MUTANTS, AND THEIR USES

BACKGROUND OF THE INVENTION

In the field of therapeutic biotechnologies, nucleic acids have proved to be a useful tool for the regulation of gene expression, medical diagnostics, biological route modulation, molecular recognition strategies or drug design. Key strategies have been developed and have proved their efficiency for several years such as antisense nucleic acids ^{1 ,2} , ribozymes and riboswitches ^3,4 and aptamers ^5,6 . However, some innovative breakthrough in the generation of aptamer-based molecules remains needed to compete with medicinal chemistry and/or biological antibodies.

Nucleic acid aptamers consist of single-stranded DNA (ssDNA) or RNA that present defined 3D structures due to their propensity to form complementary base pairs. Their ability to fold into various secondary and tertiary structures ⁷ opens the possibility to design different conformations that are capable of specific molecular recognition of their cognate targets. RNA aptamers are prone to generate more complex 3D structures than DNA aptamers and usually display a higher binding affinity and specificity ⁶ . Triple base-pairs, hydrophobic and electrostatic interactions, van der Waals forces, shape complementarity and base stacking all combine to generate folded structures and shielded active sites that determine their binding affinity and specificity. For all of these purposes, aptamers are listed among the most important classes of drug molecules and their development is facilitated by Systematic Evolution of Ligands by Exponential enrichment (SELEX) strategies ^8,9 . This efficient method of producing high-affinity of aptamers relies on the generation of a combinatorial library of oligonucleotides (around 10 ¹⁵ ). These libraries must contain a huge pool of random- sequence oligonucleotides to maximize the chances to select good candidates. As an alternative to the chemical synthesis of random oligonucleotides at the first step of aptamer selection, the engineering of DNA polymerases designed to synthesize nucleic acids (RNA and DNA) in a random fashion may become the cutting edge of SELEX. In vivo, DNA polymerases are crucial to the DNA replication and the maintenance of the genome and therefore their role is critical for the propagation of the genetic information. All DNA polymerases found in eukaryotes, prokaryotes, archaea and viruses have been classified into subfamilies according to their structure and primary sequence similarity ^10,11 . By their inherent role, replicative DNA polymerases copy the template DNA with high fidelity and consequently their native activity is of limited use for applications in modern synthetic biology, which seeks to build novel and versatile nucleic acid polymers. Indeed, DNA polymerases have an active site that is configured to incorporate the four canonical deoxyribonucleotides and to exclude 'altered' or modified nucleotides during cellular metabolism.

Representative members of family A with a known crystal structure are E. coll DNA Pol I ¹¹ and T. aquaticus DNA pol I ¹² in prokaryotes, and phage T7 ¹³ DNA pol in viruses; in eukaryotes, Pol theta (Pol Θ) takes part in the repair of DNA double-strand- breaks (DSB) in the so-called Non-Homologous End Joining (NHEJ) process) ^14_16 , pol nu (pol v) in the repair of DNA crosslinks occurring during homologous recombination (HR) and Pol gamma (Pol y) is involved in mitochondrial DNA replication ¹⁷ . Very recently, human Pol theta has been described to display a robust terminal transferase activity that is apparent when it switches between three different mechanisms during alternative end-joining (alt-EJ) ^18,19 . Indeed, human Pol theta is able to perform non- templated DNA extension, as well as instructed replication that is templated in c/ ^' s or in trans of the DSB. The same study revealed that the non-templated transferase activity is enhanced in the presence of manganese divalent ions and can be randomly combined with templated extension on the 3'-end of a nucleic acid primer.

The other family of DNA polymerases which has some significant nucleotidyltransferase activity is the pol X family, especially in eukaryotes (Pol beta (Pol β), Pol lambda (Pol λ), Pol mu (Pol μ), and TdT) ^20,21 . This activity is usually enhanced in the presence of transition metal ions. Among them, TdT (Terminal deoxynucleotidyl Transferase) is described to catalyze non-templated, random nucleotide addition at the V(D)J junctions to increase the antigen receptor diversity ^18,22,23 . On top of this property, previous studies revealed that TdT indiscriminately incorporates ribonucleotides (NTPs) and deoxyribonucleotides (dNTPs) ' but then fails to extend it beyond 4-5 nucleotides because the primer is no longer a DNA but an RNA. This observation is compatible with the known crystal structure of murine TdT where none of the protein residues seemed to act as steric barrier close to the 2' or the 3' position of the ddATP sugar ²⁵ . However, engineering TdT to make it accept an RNA primer seems quite a challenge, given the conformation of the primer in the structure of the tertiary complex, where the conformation of the primer strand is a B-DNA, contrary to what would be expected for an RNA primer (A- DNA).

NTPs differ from dNTPs only by the presence of an additional hydroxyl group in the 2'-position of the ribose. It was shown that human Pol theta incorporated the NTPs better than TdT ¹⁸ and enabled the synthesis of long polymers of RNAs, although with low yields. The joint predisposition of such DNA polymerases (Pol theta and TdT), to i) perform random nucleotides incorporation in a template-free manner and ii) the possibility to tolerate both NTPs and dNTPs and other modified nucleotides, opens the way to create a novel nucleic acid synthetic machine. This suggests that random RNA library could be enzymatically generated much easier without the need of the reverse- transcription step of DNA fragments during SELEX.

There is a need in the art for some DNA Polymerase theta mutants capable of incorporating a large diversity of nucleic analogs and generating long polymers of, nucleic acids analogs, capable of increasing the sampling as well as improving the stability, the affinity and the specificity of functional nucleic acids such as RNA aptamers.

BRIEF SUMMARY OF THE INVENTION

The invention relates to mutant DNA polymerases of the Pol theta subfamily capable of performing non-templated nucleic acid extension, or of a functional fragment of such a polymerase, methods of producing these mutant DNA polymerases, and uses and methods of using these mutant DNA polymerases.

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least one amino acid substitution at a position selected from the group consisting of: 2322, 2328, 2334, 2335, 2384, 2387 and 2391 , the indicated positions being determined by alignment with SEQ ID NO: 1. In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least one amino acid substitution selected from the group consisting of: a Proline (P) to an aliphatic amino acid or a polar amino acid substitution at position 2322, an Alanine (A) to an aliphatic amino acid or a polar amino acid substitution at position 2328, a Leucine (L) to an aliphatic amino acid substitution at position 2334, a Glutamic acid (E) to an aliphatic amino acid or a polar amino acid substitution at position 2335, a Glutamine (Q) to an aliphatic amino acid or a polar amino acid substitution at position 2384, a Tyrosine (Y) to an aromatic amino acid or an aliphatic amino acid substitution at position 2387, and a Tyrosine (Y) to an aromatic amino acid or an aliphatic amino acid substitution at position 2391 , the indicated positions being determined by alignment with SEQ ID NO: 1 .

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least one amino acid substitution at position 2322, wherein the amino acid at position 2322 is substituted by an aliphatic amino acid selected from the group consisting of: Valine (V) and Alanine (A), the indicated position being determined by alignment with SEQ ID NO: 1.

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least one amino acid substitution at position 2322, wherein the amino acid at position 2322 is substituted by a polar amino acid selected from the group consisting of: Threonine (T) and Serine (S), the indicated position being determined by alignment with SEQ ID NO: 1.

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least one amino acid substitution at position 2328, wherein the amino acid at position 2328 is substituted by an aliphatic amino acid selected from the group consisting of: Valine (V) and Glycine (G), the indicated position being determined by alignment with SEQ ID NO: 1.

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least one amino acid substitution at position 2328, wherein the amino acid at position 2328 is substituted by a polar amino acid selected from the group consisting of: Threonine (T) and Serine (S), the indicated position being determined by alignment with SEQ ID NO: 1 .

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least one amino acid substitution at position

2334, wherein the amino acid at position 2334 is substituted by an aliphatic amino acid selected from the group consisting of: Methionine (M), Isoleucine (I) and Alanine (A), the indicated position being determined by alignment with SEQ I D NO: 1 .

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least one amino acid substitution at position

2335, wherein the amino acid at position 2335 is substituted by an aliphatic amino acid selected from the group consisting of: Glycine (G) and Alanine (A), the indicated position being determined by alignment with SEQ ID NO: 1 .

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least one amino acid substitution at position 2335, wherein the amino acid at position 2335 is substituted by a polar amino acid selected from the group consisting of: Threonine (T) and Serine (S), the indicated position being determined by alignment with SEQ ID NO: 1 .

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least one amino acid substitution at position 2384, wherein the amino acid at position 2384 is substituted by an Alanine (A), the indicated position being determined by alignment with SEQ I D NO: 1 .

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least one amino acid substitution at position 2384, wherein the amino acid at position 2384 is substituted by a polar amino acid selected from the group consisting of: Asparagine (N), Serine (S) and Threonine (T), the indicated position being determined by alignment with SEQ I D NO: 1

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least one amino acid substitution at position 2387, wherein the amino acid at position 2387 is substituted by an aromatic amino acid selected from the group consisting of: Phenylalanine (F) and Tryptophan (W), the indicated position being determined by alignment with SEQ I D NO: 1 .

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least one amino acid substitution at position 2387, wherein the amino acid at position 2387 is substituted by an aliphatic amino acid selected from the group consisting of: Alanine (A) and Valine (V), the indicated position being determined by alignment with SEQ ID NO: 1.

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least one amino acid substitution at position 2391 , wherein the amino acid at position 2391 is substituted by an aromatic amino acid selected from the group consisting of: Phenylalanine (F) and Tryptophan (W), the indicated position being determined by alignment with SEQ ID NO: 1 .

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least one amino acid substitution at position 2391 , wherein the amino acid at position 2391 is substituted by an aliphatic amino acid selected from the group consisting of: Alanine (A) and Valine (V), the indicated position being determined by alignment with SEQ ID NO: 1.

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least one amino acid substitution selected from the group consisting of: P to V substitution at position 2322 (P2322V), A to V substitution at position 2328 (A2328V), L to M substitution at position 2334 (L2334M), E to G substitution at position 2335 (E2335G), Q to N at position 2384 (Q2384N), Y to F substitution at position 2387 (Y2387F); and Y to F substitution at position 2391 (Y2391 F), the indicated positions being determined by alignment with SEQ ID NO: 1 .

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least one amino acid substitution selected from the group consisting of: P to V substitution at position 2322 (P2322V), A to V substitution at position 2328 (A2328V), L to M substitution at position 2334 (L2334M), E to G substitution at position 2335 (E2335G), and Y to F substitution at position 2387 (Y2387F), the indicated positions being determined by alignment with SEQ ID NO: 1 .

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least two amino acid substitutions at positions 2334 and 2335, the indicated position being determined by alignment with SEQ ID NO: 1 . In various embodiments, the amino acid at position 2334 is substituted by an amino acid selected from the group consisting of: Methionine (M), Isoleucine (I) and Alanine (A), and preferably by a Methionine (M). In some embodiments, the amino acid at position 2335 is substituted by an amino acid selected from the group consisting of: Glycine (G), Alanine (A), Threonine (T) and Serine (S), and preferably by a Glycine (G).

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises a double amino acid substitution L2334M and E2335G, the indicated positions being determined by alignment with SEQ ID NO: 1 .

In various embodiments, the invention encompasses nucleic acids comprising a nucleotide sequence encoding the mutant DNA polymerase or a functional fragment thereof, a DNA vector comprising these nucleic acids, and a host cell comprising these vectors.

In various embodiments, the invention encompasses a method for generating a mutant DNA polymerase of the Pol theta subfamily or a functional fragment thereof, comprising substituting at least one amino acid in a DNA polymerase of the Pol theta subfamily at a position selected from the group consisting of: 2322, 2328, 2334, 2335, 2384, 2387 and 2391 , the indicated positions being determined by alignment with SEQ ID NO: 1 .

In various embodiments, the invention encompasses the use of the nucleic acids, vectors, and host cells of the invention for producing a mutant DNA polymerase or a functional fragment thereof.

In various embodiments, the invention encompasses a method for producing a mutant DNA polymerase culturing the host cell of the invention under culture conditions allowing expression of the polynucleotide encoding said mutant, and optionally recovering said mutant thus expressed from the medium culture or host cells.

In one embodiment, the invention encompasses a method for incorporating nucleotides in a template-free manner comprising incubating the mutant DNA polymerase of the invention or a functional fragment thereof with nucleotide triphosphates under conditions that allow nucleotide incorporation in the absence of a template.

In one embodiment, the invention encompasses the use the mutant DNA polymerase of the invention or a functional fragment thereof, for incorporating nucleotides in a template-free manner.

In various embodiments, the invention encompasses a method for producing degenerate or random nucleotide sequences comprising incubating the mutant DNA polymerase of the invention or a functional fragment thereof with nucleotide triphosphates under conditions that allow degenerate or random nucleotide incorporation to produce degenerate or random nucleotide sequences. In some embodiments, a fixed nucleotide sequence can be added to the 3' end of the degenerate or random nucleotide sequences. In some embodiments, the degenerate or random nucleotide sequences can be amplified. In some embodiments, the amplified sequences can be cloned into a vector to generate a library of degenerate or random nucleotide sequences.

In one embodiment, the invention encompasses use the mutant DNA polymerase of the invention or a functional fragment thereof for generating an aptamer library.

In various embodiments, the invention encompasses a kit for generating an aptamer library comprising the mutant DNA polymerase of the invention or a functional fragment thereof. In some embodiments, the kit comprises reagents for degenerate or random nucleotide incorporation. In some embodiments, the kit comprises a vector for generating a library of degenerate or random nucleotide sequences.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 A-B. Sequence alignment of the Finger subdomain (residues 2333- 2474) of Pol theta (PolO) and the related A family polymerases.

(a) Strictly conserved residues are highlighted and the conserved motifs are boxed rectangle. Stars indicate the strictly conserved catalytic aspartate (D2330 and D2540) and glutamate (E2541 ) of the palm subdomain that coordinate divalent cation in human pol theta (pol 0) (Zahn et al. 2015). Triangles indicate the conserved residues that were mutated in this study and those that are potentially mutable to confer nucleotidyltransferase activity to the human pol theta (pol 0). Secondary structures based on the PDB id. (4X0P) of the human pol theta (pol 0) are depicted on the top of the multiple alignment. Human pol theta (UniProtKB/Swiss-Prot accession number 075417.2; SEQ ID NO: 1 ); Mouse pol theta (UniProtKB/Swiss-Prot accession number Q8CGS6.2; SEQ ID NO: 2); Zebrafish pol theta (NCBI accession number XP_021329106.1 ; SEQ ID NO: 3); Fruit fly mus308 pol theta 'UniProtKB/Swiss-Prot: 018475.1 ; SEQ ID NO: 4); Human pol nu (UniProtKB/Swiss-Prot accession number Q7Z5Q5.2; SEQ ID NO: 5); Mouse pol nu (UniProtKB/Swiss-Prot accession number Q7TQ07.2; SEQ ID NO: 6); Taq pol I (GenBank accession number BAA06033.1 ; SEQ ID NO: 7); Geobacillus stearothermophilus DNA pol I (GenBank: AAC37139.1 ; SEQ ID NO: 8); E. coli DNA pol Klenow fragment (PDB accession number 1 D8Y_A; SEQ ID NO: 9); E. coli DNA pol (UniProtKB/Swiss-Prot accession number P00582.1 ; SEQ ID NO: 10).

(b) Sequence comparison of the motif A of Taq pol I and human pol theta (pol 0), the immutable residue D610 is mentioned while the residues that tolerate a wide spectrum of substitution are displayed according to the nature of amino acid that they accept. Taq pol I motif A (LLVALDYSOIELRVLAH; SEQ ID NO: 1 1 ). Human pol theta motif A (SILAADYSQLELRILAH; SEQ ID NO: 12).

Figure 2A-C. Structure of human pol theta (pol 0) polymerase domain (residues 1792-2590).

(a) Surface representation of human pol theta (pol 0) (PDB 4X0P) showing the folding pattern of a right hand with the three subdomains: the fingers, the palm and the thumb. A DNA primer is inserted between the palm and the fingers and where it will be elongated in the catalytic site. The N-terminal exo-like domain is also displayed.

(b) Superposition of Taq pol I (1 QSY) and human pol theta (polO) (4X0P) where both enzymes are shown in stick models. A zoom-in stereo view of the fingers subdomain displays the residues E615 (1 QSY) compared to E2335 (4X0P) in the closed proximity of the incoming nucleotide (ddATP) facing the DNA primers (for 4X0P and for 1 QSY).

(c) Ribbon diagram of the ddATP-Ca ²⁺ structure of the finger subdomain around the catalytic site. The strictly conserved carboxylates (D2330, D2540 and E2541 ) coordinating metal cations, the mutable residues (L2334, E2335, Q2384, Y2387 and Y2391 ) are displayed as sticks. The incoming ddATP is showed in ball-and-stick models and the ssDNA primer (3'-end) is showed.

Figure 3. Deoxynucleotidyltransferase activity of human pol theta (pol 0)WT and its respective mutants. Denaturing gel showing pol 0 variants presence of Mn ²⁺ cations and each of the four dNTPs (A, T, C, G) and the mix (N). A 14-mer ssDNA (5TACGCATTAGCATA; SEQ ID NO: 13) serves as a primer being extended and forming long homo- or heteropolymers that reach up to 150-200nt. The primer extension of three mutants (NM1 1 , CS13 and GC10) are also displayed in the same conditions as the pol theta (pol 0) WT. Reactions were stopped after 30 min of incubation.

Figure 4A-D. Ribonucleotidyltransferase activity of human theta (pol 0)WT and its respective mutants.

(a) and (c) Denaturing gel showing pol O variants presence of Mn ²⁺ cations and each of the four NTPs (A, U, C, G at 0.5mM each) and the mix (N, at 0.5mM each). A 14-mer ssDNA (5TACGCATTAGCATA; SEQ ID NO: 13) serves as a primer being extended and forming long homo- or heteropolymers that reach up to 150-200nt. The primer extension of four mutants: NM1 1 , CS13, GC10 and DW9 (a) or five mutants: NM1 1 , CS13, GC10, DW9 and MC15 (c) are also displayed in the same conditions as the pol OWT. Reactions were stopped after 30 min of incubation.

(b) and (d) Time-course of a 14-mer ssDNA primer (5TACGCATTAGCATA;

SEQ ID NO: 13) by CS13 mutant in the presence of a stoichiometric mix of the four NTPs (0.5mM each). At each indicated time: 0s to 30min (b) or 0s to 60min (d), the reaction was stopped by the addition of formamide blue.

Figure 5A-C. Ribbon representations of the nucleotide binding pocket of the human pol theta (pol O) polymerase domain (4X0P).

(a) ddATP-Ca ²⁺ -pol theta (polO)WT structure showing the position of E2335 towards the incoming nucleotide.

(b) Theoretical model of ATP-Mn ²⁺ -pol theta (polO)WT structure depicting the steric hindrance between E2335 residue and the sugar moiety of the nucleotide. (c) Theoretical model of ATP-Mn - pol theta (polO)E2335G structure showing the spacing of the nucleotide binding pocket when the glutamate residue is substituted by a glycine residue.

Figure 6A-J. HPLC fragmentation of the ribonucleosides obtained after enzymatic hydrolysis of synthetic RNAs.

(a) and (f) Chromatogram of the standards solutions of the four ribonucleosides (adenosine, guanosine, uridine and cytidine at concentrations of 0.1 mM (a) or 0.25 mM (f) in digestion buffer), the retention times and the base corresponding to each peak are displayed on top of each peak.

(b) and (i) RNA hydrolysate obtained from an equimolar mix of ATP and CTP.

(c) and (j) RNA hydrolysate obtained from an equimolar mix of GTP and UTP.

(d) and (g) RNA hydrolysate obtained from an equimolar mix of the four NTPs.

(e) and (h) RNA hydrolysate obtained from a mix containing ATP/CTP/GTP/UTP at a molar ratio of 1 :1 :1 :10.

Figure 7A-D. TruSeq statistical analysis 1 : Occurences of the reads in each synthesis condition illustrated by a log-log scatter plot chart.

(a) Condition 'N', mix of four nucleotides at a ratio of 1 :1 :1 :1 (500 μΜ each).

(b) Condition Ί 01Γ with 500 μΜ of ATP, CTP and GTP, and 5 mM of UTP (ratio of 1 :1 :1 :10).

(c) Condition '5U5C with 500 μΜ of ATP and GTP and 2.5 mM of CTP and UTP

(ratio of 1 :1 :5:5).

(d) Condition '5U' with 500 μΜ of ATP, CTP and GTP and 2.5 mM of UTP (ratio of 1 :1 :1 :5).

Figure 8A-D. TruSeq statistical analysis 2: Nucleotide frequency per read represented by a box plot chart.

(a) Condition 'N', mix of four nucleotides at a ratio of 1 :1 :1 :1 (500 μΜ each).

(b) Condition '10U' with 500 μΜ of ATP, CTP and GTP, and 5 mM of UTP (ratio of 1 :1 :1 :10).

(c) Condition '5U5C with 500 μΜ of ATP and GTP and 2.5 mM of CTP and UTP (ratio of 1 :1 :5:5). (d) Condition '51T with 500 μΜ of ATP, CTP and GTP and 2.5 mM of UTP (ratio of 1 :1 :1 :5).

Figure 9A-D. TruSeq statistical analysis 3: Nucleotide proportion per incorporation cycle illustrated by a stacked bars chart.

(a) Condition 'N', mix of four nucleotides at a ratio of 1 :1 :1 :1 (500 μΜ each).

(b) Condition Ί 01Γ with 500 μΜ of ATP, CTP and GTP, and 5 mM of UTP (ratio of 1 :1 :1 :10).

(c) Condition '5U5C with 500 μΜ of ATP and GTP and 2.5 mM of CTP and UTP (ratio of 1 :1 :5:5).

(d) Condition '5U' with 500 μΜ of ATP, CTP and GTP and 2.5 mM of UTP (ratio of 1 :1 :1 :5).

Figure 10A-D. TruSeq statistical analysis 4: Nucleotide transition matrix illustrating the proportion of A/C/G/U added after a given nucleotide, horizontal stacked bars chart.

(a) Condition 'N', mix of four nucleotides at a ratio of 1 :1 :1 :1 (500 μΜ each).

(b) Condition '10U' with 500 μΜ of ATP, CTP and GTP, and 5 mM of UTP (ratio of 1 :1 :1 :10).

(c) Condition '5U5C' with 500 μΜ of ATP and GTP and 2.5 mM of CTP and UTP (ratio of 1 :1 :5:5).

(d) Condition '5U' with 500 μΜ of ATP, CTP and GTP and 2.5 mM of UTP (ratio of 1 :1 :1 :5).

Figure 11. Incorporation of modified nucleotides by pol theta (polO) CS13.

The following analogs were successfully accepted and can be decomposed in two classes; those that made polymers: (1 ) 2'-Fluoro-dUTP, (2) 2'-Fluoro-dATP, (3) 2'- Fluoro-dCTP, (4) 2'-Fluoro-dGTP, (5) 2'-Fluoro-dTTP, (6) 2'-Amino-dATP, (7) 5-methyl- UTP, (8) Ara-ATP (Vidarabine triphosphate), (9) Ara-CTP (Cytarabine triphosphate), (10) 2'-0-methyl-ATP, (1 1 ) 2'-0-methyl-CTP, (12) epsilon(e)-ATP, (13) 2-Aminopurine, (14) FANA, (15) 5EUTP, (16) Control without NTP; and those that stopped after one incorporation (because of the lack of a 3ΌΗ and/or the presence of a modified or blocked 3' group): (17) 0-CH3-ddTTP, (18) 2'-Amino-dTTP, (19) 3'-Amino-ddGTP, (20) N3-ddGTP, (21 ) N3-ddTTP, (22) 3'-deoxy-ATP, (23) 3'-deoxy-UTP, (24) 3'-deoxy-CTP, (25) 3'-deoxy-GTP, (26) 3'-deoxy-NTP. The reactions were performed in the same conditions as with the natural NTPs in presence of Mn ²⁺ using a 14-mer ssDNA primer (5'TACG C ATTAG C ATA ; SEQ ID NO: 13).

Figure 12. Distribution of the ribonucleoside as function of the composition of ribonucleotide substrate added to CS13 mutant.

Figure 13. Ligation of a constant region to the 3 -end of the RNA pool synthesized by pol 0-DW9 mutant and by using T4 RNA ligase. Time-course of the elongation of a 15-mer ssRNA primer by DW9 mutant in the presence of a stoichiometric mix of the four NTPs (0.5 mM each) and separated in a denaturing 8% acrylamide gel. At each indicated time (5s, 30s, 1 min, 5min and 15 min), the reaction was stopped by the addition of formamide blue. (1 ) Control: RNA primer + NgRNA without enzyme. (2) Elongation control after 5 sec. (3) Elongation control after 30 sec. (4) Elongation control after 1 min. (5) Elongation control after 5 min. (6) Elongation control after 15 min. (7) Ligation after 5 sec-elongation (+15). (8) Ligation after 30 sec- elongation (+15). (9). Ligation after 1 min-elongation (+15). (10). Ligation after 5 min- elongation (+15). (1 1 ). Ligation after 15 min-elongation (+15). (12) Autoligation control.

(AlexaFluor488) The fluorescence signal displays the presence and the elongation of all the RNA fragments containing the 5'Alexa-fluor-labelled RNA primer (AlexaFluor488-UACGCAUUAGCAAUG; SEQ ID NO: 14).

(CY5) The fluorescence signal displays the fragments that have been ligated with the constant region (oligonucleotide NgRNA-Cy5 5 -UUAUGCUAAUGUCCC-3'- CY5; SEQ ID NO: 15) at 3'-end by T4 RNA ligase.

(Merged) The Alexa fluorand CY5signals have been merged to better evaluate the quality of the reactions (elongation + ligation).

Figure 14. Incorporation of modified nucleotides by pol theta (pole) WT in comparison with the pole CS13. The following analogs were tested for the elongation of the ssDNA primer (5TACGCATTAGCATA; SEQ ID NO: 13) by pole CS13 compared to pole. (1 ) 2'-Amino-dATP, (2) 2'-Amino-dUTP, (3) 2'-Amino-dCTP, (4) 2'-Amino-dGTP, (5) mix of 2'-Amino-dATP/dUTP/dCTP/dGTP, (6) 2'-0-methyl- dATP, (7) 2'-0-methyl-dUTP, (8) 2'-0-methyl-dCTP, (9) 2'-0-methyl-dGTP, (10) mix of 2'-0-methyl-dATP/dUTP/dCTP/dGTP, (1 1 ) 2'-azido-2'-dATP (12) 2'-azido-2'-dUTP, (13) 2'-azido-2'-dCTP _! (14) 2'-azido-2'-dGTP _! (15) mix of 2'-azido-2'- dATP/dUTP/dCTP/dGTP, (16) 2'-fluoro-dATP, (17) 2'-Fluoro-dUTP (18) 2'-fluoro- dCTP, (19) 2'-Fluoro-dGTP, (20) 2'-fluoro-dTTP, (21 ) mix of 2'-fluoro- dATP/dUTP/dCTP/dGTP (22) Ara-ATP (Vidarabine triphosphate), (23) Ara-CTP (Cytarabine triphosphate), (24) mix of Ara-ATP and Ara-CTP (25) epsilon(e)-ATP (26) 2-Aminopurine riboside triphosphate.The reactions were performed in the same conditions as with the natural NTPs in presence of Mn ²⁺ .

Figure 15. Incorporation of modified nucleotides by polo DW9. The following analogs were tested for the elongation of the ssDNA primer (5'TACG C ATTAG C ATA ; SEQ ID NO: 13) by polo DW9 compared to pole. (1 ) 2'- Amino-dATP, (2) 2'-Amino-dUTP, (3) 2'-Amino-dCTP, (4) 2'-Amino-dGTP, (5) mix of 2'-Amino-dATP/dUTP/dCTP/dGTP, (6) 2'-0-methyl-dATP, (7) 2'-0-methyl-dUTP, (8) 2'- O-methyl-dCTP, (9) 2'-0-methyl-dGTP, (10) mix of 2'-0-methyl- dATP/dUTP/dCTP/dGTP, (1 1 ) 2'-azido-2'-dATP (12) 2'-azido-2'-dUTP, (13) 2'-azido-2'- dCTP, (14) 2'-azido-2'-dGTP, (15) mix of 2'-azido-2'-dATP/dUTP/dCTP/dGTP, (16) 2'- fluoro-dATP, (17) 2'-Fluoro-dUTP (18) 2'-fluoro-dCTP, (19) 2'-Fluoro-dGTP, (20) 2'- fluoro-dTTP, (21 ) Ara-ATP (Vidarabine triphosphate), (22) Ara-CTP (Cytarabine triphosphate), (23) epsilon(e)-ATP (24) 2,6-diaminopurine riboside triphosphate. The reactions were performed in the same conditions as with the natural NTPs in presence of Mn ²⁺ .

DETAILED DESCRIPTION OF THE INVENTION

The application relates to the subject-matter as defined in the claims as filed and as herein described. In the application, unless specified otherwise or unless a context dictates otherwise, all the terms have their ordinary meaning in the relevant field(s).

Functional nucleic acids, in particular nucleic acid aptamers, can exhibit valuable advantages and properties compared to protein therapeutics in terms of size, synthetic accessibility, affinity and specificity. As these molecules can be selected from pools of random-sequence oligonucleotides, the engineering of DNA or RNA polymerases remains the basis of the successful enzymatic synthesis of functional nucleic acids analogs. The present invention provides A-family DNA polymerase mutants that can efficiently incorporate natural or modified nucleotides, particularly, ribonucleotides at the 3'end of a nucleic acid, resulting in long polymers that could serve as a library for the selection of functional nucleic acids, in particular aptamers or ribozymes. Five mutants of DNA polymerase (named CS13, DW9, MC15, NM1 1 and GC10) were generated and the mutations were focused on the residues located in close proximity of the catalytic site.

The functional characterization of each mutant has been performed and two promising candidates (CS13 and DW9) were able to display an enhanced efficiency to incorporate the four natural ribonucleotides (ATP, UTP, CTP and GTP) compared to the wild-type.

As a result, long homo- or heteropolymers of ribonucleotides were obtained whose length is highly variable (20-300nt) and can be controlled by the time-length of the reaction, which necessitate Mn ²⁺ ions. HPLC analysis of the resulting ribonucleosides obtained after enzymatic digestion of the newly synthesized RNA also showed the equal probability of incorporation of the four ribonucleotides and the randomness of the sequences was confirmed after RNA-sequencing. Moreover, the incorporation of modified deoxy- and ribonucleotides has been also investigated. The following analogs were successfully accepted by at least one promising mutant: 2'- Fluoro-dNTP (2'-Fluoro-dATP, 2'-Fluoro-dUTP, 2'-Fluoro-dCTP, 2'-Fluoro-dGTP, 2'- Fluoro-dTTP, and mixtures thereof); 2'-Amino-dNTP (2'-Amino-dATP, 2'-Amino-dUTP, 2'-Amino-dTTP; 2'-Amino-dCTP, 2'-Amino-dGTP, and mixtures thereof); 2 -O-methyl- dNTP (2'-0-methyl-dATP, 2'-0-methyl-dUTP, 2'-0-methyl-dCTP, 2'-0-methyl-dGTP, and mixtures thereof), 2 ^' -N ₃ -dNTP (2'-azido-2'-dATP, 2'-azido-2'-dUTP, 2'-azido-2'- dCTP, 2'-azido-2'-dGTP, and mixtures thereof); 0-CH3-ddTTP, 3'-Amino-ddGTP, N3- ddGTP, N3-ddTTP, Ara-ATP (Vidarabine triphosphate), Ara-CTP (Cytarabine triphosphate), 2'-0-methyl-ATP , 2'-0-methyl-CTP, 3'-deoxy-ATP, 3'-deoxy-UTP, 3'- deoxy-CTP, 3'-deoxy-GTP, 3'-deoxy-NTP, epsilon(e)-ATP, 2-Aminopurine riboside triphosphate, FANA (9-(2'-Fluoro-2'-deoxy-p-D-arabinofuranosyl) adenine), 5-ethynyi- UTP, and 5-methyl-UTP. These properties of DNA polymerase will contribute to broaden the applicability of chemically modified nucleic acids in RNA biology, medical diagnosis, and molecular recognition strategies. By this work, a versatile toolbox for RNA and DNA functionalization and for aptamer design may come into being.

Pol theta mutants with new properties

DNA polymerases contain an active site with highly conserved motifs that are structurally superimposable within each family. Several studies showed that most DNA polymerases share two conserved regions, motifs A and C, that are located in the palm subdomain ²⁸ . Motif A contains a strictly conserved aspartate at the junction of a beta(P)-strand and alpha(oc)-helix, while motif C contains two carboxylate residues (Asp or Glu) at a beta-turn-beta structural motif ¹⁰ . In the case of human pol theta (ροΙΘ) the strictly catalytic conserved aspartate (D2330) is located between beta12 (β12) and alpha9 (cc9) and is part of a strictly conserved motif (DYSQLELR) in the different pol theta (pol Θ) (Figure 1a). This catalytic aspartate corresponds to D610 in Taq DNA polymerase I which interacts with the incoming dNTP and stabilizes the transition state that leads to phosphodiester bond formation ²⁹ . This sequence motif (DYSQLELR) is exceptionally well conserved in A-family polymerases and within the Pol I family ¹³ as described in the Figure 1 for pol theta (pol Θ), pol nu (pol v) and bacterial pol I. Previous studies showed that mutation of the catalytic aspartate (D610) completely altered the polymerase activity and was immutable while systematic mutations of the other 13 residues (605 to 617) including the most conserved region (DYSQLELR) of the motif A of Taq DNA pol I showed that some positions tolerated a wide spectrum of substitutions (L605, L606, V607, A608, L609, S612, 1614, R617). Otherwise, the remaining residues tolerated mainly conservative substitutions (Y61 1 , Q613, E615 and L617). The residue S612 was highly mutable and accepted substitutions that are diverse in size, and hydrophobicity while keeping WT-like activity ²⁸ . Alignment of human pol theta (ροΙΘ) and Taq pol I sequences (Figure 1 b) illustrates the conservation of the DYSQLELR and displays the main residues corresponding to pol theta (ροΙΘ) sequence (D610 corresponds to D2330, D615 to D2335). The same authors described that the residues Y61 1 , Q613, E615 and L617 were involved in dNTP binding, and especially E615 that forms a hydrogen bond with Y671 (a residue located in helix O within the finger motif and stacks with the base moiety of the incoming dNTPs). The inventors follow this line of reasoning for human pol theta (pol Θ) with the aim of widening its substrate specificity to accept natural and modified NTPs. Indeed, good candidates for site-directed mutations are L2334, E2335, A2328 within the motif A and also residue Y2387 in motif B (corresponding to Y671 in Taq pol I).

The crystal structure of the polymerase domain of pol theta (pol Θ) (residues 1792-2590; PDB 4X0P) ¹⁵ ' ²⁶ reveals the same right hand-like topology seen in the bacterial and phage homologs. The surface representation of pol theta (pol Θ) (Figure 2a) illustrates the three subdomains (fingers, palm and thumb) and a N-terminal exonuclease subdomain. The DNA strand slides into the catalytic site between the fingers and the palm where the motif A residues make the junction between the two parts. When pol theta (pol Θ) is superimposed with Taq pol I (1 QSY) (Figure 2b), both structures displayed a closed overall conformation. The residues E615 (1 QSY) and E2335 (4X0P) are oriented in similar manner towards the incoming nucleotide and can interact with the sugar moiety of the nucleotide. Taq pol I has been extensively studied and diversified to increase its ability to incorporate NTPs and modified nucleotides ³⁰ . Tag pol I libraries were generated by diversifying the residues 61 1 -617 that include the nucleotide binding pocket and the steric gate residue E615. Among the different Taq pol I mutants, 1614K and a multiple-site mutant SFR3 (A597T, W604R, L605Q, 1614K, E615G) showed the ability to incorporate NTPs after short-patch compartimentalized self-replication (spCSR) selection ³⁰ while no detectable primer extension by wt- Tag pol I was observed. However, the best mutant was able to incorporate up to 6 NTPs in the presence of Mg ²⁺ and up to 14 NTPs when Mn ²⁺ is used instead of Mg ⁺⁺ .

In light of the close similarity of the described region of Taq pol I and human pol theta (pol Θ) (Figure 1 b), the inventors focused on the corresponding steric gate residue E2335 and the immediately close residue L2334. The inventors then also analyzed the residues located in the proximal region of the nucleotide binding pocket (P2322, A2328, Q2384, Y2387 and Y2391 ). The ribbon representation of human pol theta (pol Θ) (Figure 2c) depicts the spatial organization of the catalytic site with the strictly conserved carboxylates (D2330, D2540 and E2541 ) that chelate the metal ion. In the immediate proximity of the catalytic triad the residues E2335 is facing the incoming nucleotide where its carboxylate group is susceptible to interact with the C2' of the ribose of the nucleotide (ddATP) and to form also a hydrogen bond with the residue Y2387 and/or Y2391 . The role of E2335 as a steric gate may lie in the fact that a steric constrain would exist if the C2' of the ribose holds a hydroxyl group (Figure 5). In addition, the polar property of E2335 may play a role in the formation of a salt bridge. Otherwise, the residue Y2387 has been described to interact with the beta (β)- phosphate of the incoming nucleotide ¹⁵ and the aromatic cycle is stacked alongside to the nucleotide base. NTPs incorporation of the designed mutants: the pivotal role of the steric gate residue

Human pol theta (pol Θ) WT has been already described to have the ability to incorporate NTPs ¹⁸ in a template-free manner from the 3'-end of a ssDNA or ssRNA primer, albeit with a low yield. However, it is already apparent that human pol theta (pol Θ) shows an enhanced nucleotidyltransferase activity compared to Terminal deoxynucleotidyltranferase (TdT) in presence of NTPs, which stopped the elongation after 5-6 additions.

With the aim of selecting the best human pol theta (pol Θ) mutant with an enhanced incorporation of NTPs, the inventors proceeded to evaluate the ability to elongate a ssDNA primer through a nucleotidyltransferase assay. One or two-site directed mutagenesis were performed and led to generate the following mutants: NM1 1 (Y2387F), CS13 (E2335G), GC10 (P2322V), DW9 (L2334M-E2335G) and MC15 (A2328V).

Bacteria containing these mutants was deposited at the Collection Nationale de Cultures de Microorganismes (CNCM) at the Institut Pasteur, 25, Rue du Docteur Roux, 75724 Paris, FR, on September 14, 2017, under the deposit numbers CNCM I- 5238 (E.coli A1 -1791_CS13); CNCM I-5239 (E.coli A1 -1791_DW9); CNCM I-5240 (E.coli A1 -1791_MC15); and CNCM 1-5241 (E.coli Δ1 -1791_ΝΜ1 1 ).

The wt-like activity was first tested with dNTPs (Figure 3) and all the mutants tested retained the native activity as they incorporated the four natural dNTPs and generated fragments of length of up to 200 nt like the pol theta (pol Θ) WT in the presence of 5 mM of Mn When dNTPs were replaced by each NTP (ATP, UTP, CTP, GTP) the pol theta (pol 0)WT showed similar results as previously described ¹⁸ . ATP and GTP (purine bases) are well tolerated by pol 0WT and medium-length homopolymers of A and G (up to 50-70nt) were obtained. UTP and CTP were incorporated but the elongation stopped after a few additions. The mutants NM1 1 and GC10 were unable to incorporate more than 3 or 4 NTPs. Surprisingly, the mutant CS13 was particularly efficient as more than 50 of each ribonucleotide were readily incorporated, except for UTP that formed homopolymers of U whose length reached only up to 15nt, still longer than the pol 0WT would add. Furthermore, when the four NTPs were mixed together the elongation was clearly enhanced compared to the wild- type enzyme, in that 100% of the primer were extended and the synthesized heteropolymers reached more than 200 nt in 30 min (Figure 4a and 4c). Mutants DW9 and MC15 were also tested for their incorporation rate of NTP: MC15 did not show better efficiency than the wild-type (Figure 4c) while DW9 showed an efficiency comparable to CS13 mutant (Figure 4a and 4c). Overall the best mutants are CS13 and DW9 which both carry the E2335G mutation, with a significantly altered substrate specificity towards ribonucleotides and with an enhanced processivity of RNA molecule extension compared to the pol 0WT. It also opens the way to investigate the incorporation of modified nucleotides by the same mutants.

In the perspective of generating a library of random sequences of RNA with controlled fragment lengths the kinetics of primer extension was performed in the same conditions and allowed to monitor the length of the products as a function of the reaction time. In the case of 20-30nt RNA fragments, 1 min-reaction is sufficient to complete the synthesis, which shortens significantly the SELEX process of aptamer assembly (Figure 4b and 4d).

Structural model of incorporation of ribonucleotide by CS13 mutant

CS13 mutant exhibited an efficient (ribo)nucleotidyltransferase activity by extending a 14-mer ssDNA primer in the presence of Mn ²⁺ . This novel attribute is provided by the substitution of the charged glutamate residue E2335 by the small and flexible glycine residue. The steric gate residue E2335 is located in close proximity of the sugar moiety of the incoming nucleotide. In the case of ddATP-pol theta (pol Θ) structure (4X0P PDB code) the carboxylate group of the glutamate does not interfere with the positioning of the nucleotide (Figure 5a). In the case of NTP, the presence of the hydroxyl group at the C2' position creates both steric hindrance and electrostatic constraints between the sugar moiety and the pocket shaped by the residues E2335, Y2387 and Y2391 (Figures 2c and 5b). The mutation E2335G obviously enlarges the nucleotide binding pocket and is indeed compatible with an increased ability to incorporate a ribonucleotide into the 3'-end of the ssDNA primer (Figure 5c). Similar observations were described for E. coli phage T7 DNA polymerase, the ribose moiety of the incoming nucleotide is lodged between the aromatic ring of the residue Y526 and the aliphatic carbons of the E480 side chain, which itself is hydrogen-bonded to the hydroxyl of the strictly conserved Y530 ¹³ .The spatial orientation of these residues forms a hydrophobic pocket at the C2' position of the ribose that might exclude ribonucleotides from the active site, thus providing a structural basis for the strong discrimination against ribonucleotide incorporation by DNA polymerases ¹³ .

CS13 mutant incorporates equally the four ribonucleotides and generates a random-sequence library of ssRNA.

The success of a SELEX method relies on the quality of the initial nucleic acids library. Therefore, the possibility to access to a large collection of random-sequence RNA or DNA fragments is essential. The CS13 mutant demonstrated an outstanding ability to elongate DNA primer with ribonucleotides. In order to confirm its added value, the incorporation rate of the four NTPs has been evaluated by HPLC analysis of the overall base composition of the newly synthesized RNAs. The RNAs were completely digested into ribonucleosides according to a published protocol ²⁷ before HPLC separation. The chromatogram of the standards solutions of the four ribonucleosides (adenosine, guanosine, uridine and cytidine at concentrations of 0.1 mM or 0.25 mM in the digestion buffer) indicated four peaks eluted at 5.55-5.60 min, 6.36-6.39 min, 7.82- 7.90 min, 9.74-9.75 min respectively for cytidine, uridine, guanosine and finally adenosine (Figure 6a and 6f). The inventors were interested in determining whether the composition of the initial ribonucleotide substrates NTP in the elongation reaction mix had an effect on the propensity of the polymerase to incorporate one ribonucleotide at the expense of another. The denaturing gels of the products generated by the CS13 mutant (Figure 4a and 4c) indicated that in presence of an equal proportion of the four NTPs (1 :1 :1 :1 at 0.5mM each), long polymers of RNA were synthesised but they give no indication on the distribution of each NTP along the sequence. However, the incorporation of each of the four NTPs displayed different trends when used at 0.1 mM each as the UTP seemed to be less integrated by the enzyme. These differences were not observed any more when using the four nucleotides at 0.25 mM each. Thus, the reaction products from NTPs (1 :1 :1 :1 ) were cleaned up and hydrolysed, and the resulting ribonucleosides were analysed by HPLC (Figure 6d and 6g). In addition, another synthesis condition has been prepared containing tenfold UTP (1 :1 :1 :10) and analysed in the same way (Figure 6e and 6h). For both conditions, four peaks were observed, corresponding to the retention time of the four nucleosides, but the calculated peak areas of each component revealed different distributions (Table I, Figure 1 ). The initial mix of 1 :1 :1 :1 NTPs resulted in a global scattering of 21 % _m0 i C, 41 % _m0 i G, 16% _m0 i A and 19% _m0 i U (Figure 6d) or 24.8% _m0 i C, 24.6% _m0 i G, 22.1 % _m0 i A and 28.6% _m0 i U (Figure 6g), whereas the initial mix of 1 :1 :1 :10 NTPs gave 18% _m0 i C, 24% _m0 i G, 14% _m0 i A and 49% _m0 i U (figure 6e) or 7.6% _m0 i C, 9.0% _mO i G, 7.6% _m0 i A and 75.9% _m0 i U (figure 6h). This result highlighted the possibility to modify the initial substrate composition to modulate or favour the incorporation of one or several specified nucleotide(s) in the final products. Moreover, it is worth noting that the equimolar combination of only two NTPs (A/C; Figure 6b and 6i and U/G; Figure 6c and 6j) exhibited an equal probability of incorporation of both substrates (approximately 49% _m oi C, 47% _m0 i A and 57% _m0 i G, 34% _m0 i U (figure 6c); approximately 55.8% _m0 i C, 44.2% _m0 i A and 51 .9% _m0 i G, 48.1 % _m0 i U (figure 6j).

In summary HPLC analyses helped to confirm that the CS13 mutant accepts roughly equally the four natural NTPs, ensuring the randomness of the sequence of each synthesized fragment.

The sequencing of RNA products revealed more details about the ribonucleotides incorporation behaviour of the mutant CS13. For the different conditions tested the RNA library displayed between 5 to 14 millions of reads. For the whole library, the occurrence of the different sequences was estimated and was plotted as illustrated in the figure 7. Among 12,733,722 reads in the sample 'Ν', 8,766,652 reads had unique sequence (occurrence =1 ), representing almost 70% of the total reads in the library (Figure 7a). The rest being dispersed between 2 (1 ,441 ,628 other reads with sequences repeated twice) and 2000 occurences (1 sequence). When the ratio of UTP has been increased in the synthesis mix (samples Ί 01 , '51T and '5C5U') the number of unique reads drastically decreased, multiplying the number of repeated reads. Anyway, the majority of the reads had a moderate number of occurrences ranged between 2 to 100 for the three conditions (Figures 7b, 7c and 7d). These results demonstrated the randomness of each RNA pool and insured a starting library size of 10 ⁶ -10 ⁷ , that represents the typical size used for SELEX ³¹ . Another statistical parameter that has been evaluated is the frequency of each ribonucleotide per read and per incorporation cycle (Figures 8 and 9). Globally, the four synthesis conditions indicated a slightly equivalent frequency (20-25% each; 26,7% A, 25%C, 24% G et 24,3%U) for the four nucleotides. Within the 65 cycles, the global proportion of added nucleotide remained constant with an increased value for UTP when an excess of UTP has been added ('10U' and '5U') to the detriment of CTP incorporation (Figures 9b and 9c). When CTP is 5-fold concentrated ('5C5U'), the amount of CTP incorporation has been also increased.

Finally, the probability of adding one (A/C/G/U) nucleotide in position N after a given nucleotide in position N-1 has been estimated for the four same conditions. This parameter might indicate the frequency of forming dinucleotides in each sequence. Among the 16 possible dinucleotides, GC seemed to be the most plausible when an equimolar ratio of ribonucleotides is present, to the opposite, it is less probable to form UC and GG dinucleotides in the same case (Figure 10a). The increase of UTP amount in the other conditions seemed to increase the probability of incorporation of U after a U (UU dinucleotide) only when it is present in a 10-fold excess (Figure 10b). When UTP and CTP were present in 5-fold excess (Figure 10c), an equilibrium of the transition behaviour took place for all the nucleotides (every scenario is possible). Lastly, a 5-fold increase of UTP amount (Figure 10d) appeared to promote the formation of GU dinucleotide compared to the condition 'N'. Put together, these results proved the randomness of the RNA pool synthesis and the suitable size of the generated libraries, which validates their use for SELEX procedures. Modified nucleotides incorporation by the CS13 and DW9 mutants

The major drawback of aptamers is their relative instability and sensitivity to hydrolysis in biological fluids. The solution to bypass this point is to produce nuclease- resistant RNA molecules by different approaches ^32-34 . Modifications can be attempted on the nucleotide sugar moiety, the phosphodiester covalent link or on the base.

The incorporation of modified deoxy- and ribonucleotides has been also investigated in this study. The following analogs were successfully accepted and resulted in long polymers: 2'-Fluoro-dNTP (2'-Fluoro-dUTP, 2'-Fluoro-dATP, 2'-Fluoro- dCTP, 2'-Fluoro-dGTP, 2'-Fluoro-dTTP, and mixtures thereof), 2'-Amino-dNTP (2'- Amino-dATP, 2'-Amino-dCTP, 2'-Amino-dGTP, 2'-Amino-dUTP, 2'-Amino-dTTP, and mixtures thereof), 2 -O-methyl-dNTP (2'-0-methyl-dATP, 2'-0-methyl-dUTP, 2'-0- methyl-dCTP, 2 -O-methyl-dGTP, and mixtures thereof), 2'-azido-2'-dNTP (2'-azido-2'- dATP, 2'-azido-2'-dUTP, 2'-azido-2'-dCTP, 2'-azido-2'-dGTP, and mixtures thereof), 5- methyl-UTP, Ara-ATP (Vidarabine triphosphate), Ara-CTP (Cytarabine triphosphate), 2'-0-methyl-ATP, 2'-0-methyl-CTP, epsilon(e)-ATP, 2-Aminopurine, FANA, 5-ethynyl- UTP.

All the 2'-fluoro (Figure 11 , lanes 1 -5; figure 14, lanes 16 to 21 ) and 2'-amino modified nucleotides (Figure 11 , lane 9; figure 14, lanes 1 to 15) were efficiently incorporated (Figure 11 and Figure 14). It has been described that the incorporation rate of fluoro-modified ribonucleotides by T7 RNA polymerase was ten-fold lower than that of the natural substrates ³³ . CS13 mutant depicted a high tolerance for these modified substrates as long homopolymers (up to 200nt) were synthesized in the same experimental conditions as the primer extension by natural NTPs or dNTPs, with a much greater efficiency than pol Θ WT. This modification is suitable for ribozyme development and aptamers selection as the fluoro-modified oligonucleotides have better ribonucleases resistance ^35,36 . The incorporation of 2'-fluoro modified nucleotides by DNA polymerases were already assessed in several studies ³⁷ and the incorporation rate of modified nucleotides by enzymatic synthesis remains relatively limited compared to the chemical synthesis. In that way, many efforts are still being focused on the substrate specificity of DNA polymerases to benefit from the attractive properties of different types of modified oligonucleotides.

The CS13 mutant failed to elongate the ssDNA primer in presence of 2' O-

Methyl modified nucleotides further than just a few nucleotides (Figure 1 1 , lanes 13 and 14; figure 14, lanes 6 to 10). 2'-0-methyl-modified RNA display better stability against base hydrolysis and ribonucleases as well as increased Tm values. This attribute may be useful to preserve the 3D conformation and to widen the diversity of a functional aptamers ³⁸ .

The incorporation of 2'-amino dATP and 2'-amino dGTP (Figure 14, lanes 1 -5) appeared to be qui efficient with the CS13 mutant, more than the 2'-amino dUTP and 2'-amino dCTP. For 2'-N ₃ (azido) dNTP, the incorporation was very good with the mutant CS13, almost as good as the 2'-F derivatives, allowing the possibility to form click chemistry experiments with compounds containing an alkyne group (Figure 14, lanes 1 1 -15).

Etheno-ATP (Figure 1 1 , lane 15; Figure 14, lane 25) formed also long polymers with CS13 mutant. This base-modified nucleotide is fluorescent in solution and is often used in fluorescence-quenching application or FRET-studies on conformational changes.

Otherwise, FANA (Figure 11 , lane 17) (2-deoxy-2-fluoroarabinonucleotide) seemed to be accepted by CS13 mutant but it failed to elongate primers into long polymers. It has been described that FANA-aptamers showed greater thermal stability, nuclease resistance and a stronger binding into their targets such as thrombin or, HIV- 1 reverse transcriptase ³⁹ for which the binding affinity is in the picomolar range and demonstrated an efficient activity in vitro. Two fluorescent analogues of adenine nucleotides were tested: etheno-adenine (Figure 14, lane 25; Figure 1 1 , lane 12) and 2-aminopurine (Figure 14, lane 26; Figure 11 , lane 13). Both were incorporated in CS13 mutant but formed only short polymers.

For the modified nucleotides that were weakly incorporated by CS13 mutants

(FANA, 5-ethynyl-UTP, 5-methyl-UTP, Ara-NTP) different reaction conditions (buffer, metal ions, pH) and other mutants like Y2387F (NM1 1 ) can be tested that can widen substrate specificity depending on their modifications. DW9 mutant (L2334M-E2335G) has been already tested with the same panel of modified nucleotides but showed the same behavior as CS13 mutant. Moreover, hypotheses of inventors explaining how CS13 mutant accept well ribonucleotides and some modified nucleotides are based on a structural model where ddATP was replaced by ATP and the glutamate residue was just replaced by glycine. To corroborate this model, the crystal structure of the mutant CS13 can be performed so to visualize the spatial rearrangements of the nucleotide binding pocket.

Similar results were observed for the DW9 mutant for the 2'-Amino, 2'-Fluoro and 2'-Azido nucleotides as longs polymers were formed (Figure 15; respectively lanes 1 -5, lanes 1 1 -15 and lanes 16-20). Otherwise, an enhanced incorporation of 2'- O-methyl was observed with the DW9 mutant compared to CS13 mutant (Figure 15; lanes 6-10). In addition, a better incorporation of epsilon-ATP (Figure 15; lane 23), can be noticed compared to CS13 mutant while DW9 failed to incorporate Ara-ATP and Ara-CTP nucleotides (lanes 21 and 22).

Building RNA aptamer libraries for SELEX

Human pol theta (pol 0)-CS13 and DW9 mutants demonstrated reliable ability to perform RNA random synthesis and to incorporate a large panel of modified nucleotides. The processivity and the randomness of its activity have been assessed quantitatively and this new enzymatic machinery should be useful for therapeutics applications. In that way, the continuous quest for ultra-effective, selective and nontoxic nucleic acids-based drugs remains the driving force of aptamer design strategies. This work offers a viable biological alternative to the generation of RNA random sequences by chemical synthesis and library design. By establishing an efficient enzymatic SELEX procedure, the assay costs will be reduced at the same time as the duration of the selection cycle. The resulted pool of RNAs obtained with CS13 and DW9 mutants served as starting candidates for aptamer library. For that purpose, a fixed fragment of RNA was added to the end of each synthesized RNA. This fragment can serve as matrix strand to amplify the selected aptamer after each cycle of SELEX. The inventors decided to implement a ligation of the fixed fragment to each synthesized RNA by exploiting T4 RNA ligase I activity. The results show that the ligation of these fixed oligonucleotides occurs (Figure 13), thus demonstrating that it is possible to add a constant region to the synthetic RNA pool and that the RNA library can hence be screened or amplified for different applications, in particular for aptamer selection.

Mutant DNA polymerases of the Pol theta family

The invention relates to mutant DNA polymerases of the Pol theta subfamily capable of performing non-templated nucleic acid extension, or of a functional (i.e., capable of performing non-templated nucleic acid extension) fragment of such a polymerase, methods of producing these mutant DNA polymerases, and uses and methods of using these mutant DNA polymerases.

The term "DNA polymerase theta" or "pol theta" or "polO" refers to a protein encoded by the POLQ gene in mammalian genome. This low-fidelity DNA polymerase is involved in a DSB (Double Strand Breaks) repair pathway termed "alternative End- joining" (alt-EJ) of "Theta-mediated end-joining". This pathway is characterized by the joining of the 3' single-stranded DNA tails which occurs when a DNA break cannot be efficiently repaired by Ku-dependent non-homologous end joining ⁴⁰ . The polymerase is able to efficiently replicate through an abasic site by functioning both as a mispair inserter and as a mispair extender ⁴¹ . DNA polymerase theta has a characteristic C- terminal DNA polymerase domain linked via a central region to a N-terminal DNA- helicase-like domain ^28,42 . Pol theta has the ability to switch templates and prime from homologies but also it can extent some single-stranded DNA substrates ^43,44 . When manganese cations (Mn ²⁺ ) are present is physiological concentrations pol theta does not have template-independent terminal transferase activity ²⁸ whereas with high ratios of Mn ²⁺ aver Mg ²⁺ the polymerase appears to trigger template-independent extension. Mn ²⁺ ions have been shown to relax template specificity in many polymerases ⁴⁵ . Representative examples of pol theta include without limitation, human (Gene ID.10721 ), rat (Gene ID. 288079), chicken (Gene ID.418326), canine (Gene ID.488003), Zebrafish (Gene ID. 566079), Fruit fly mus308 (Gene ID. 41571 ) and mouse (Gene ID. 77782) forms.

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least one amino acid substitution at a position selected from the group consisting of: 2322, 2328, 2334, 2335, 2384, 2387 and 2391 , the indicated positions being determined by alignment with SEQ ID NO: 1.

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least one amino acid substitution selected from the group consisting of: a Proline (P) to an aliphatic amino acid or a polar amino acid substitution at position 2322, an Alanine (A) to an aliphatic amino acid or a polar amino acid substitution at position 2328, a Leucine (L) to an aliphatic amino acid substitution at position 2334, a Glutamic acid (E) to an aliphatic amino acid or a polar amino acid substitution at position 2335, a Glutamine (Q) to an aliphatic amino acid or a polar amino acid substitution at position 2384, a Tyrosine (Y) to an aromatic amino acid or an aliphatic amino acid substitution at position 2387, and a Tyrosine (Y) to an aromatic amino acid or an aliphatic amino acid substitution at position 2391 , the indicated positions being determined by alignment with SEQ ID NO: 1 .

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least one amino acid substitution at position 2322, wherein the amino acid at position 2322 is substituted by an aliphatic amino acid selected from the group consisting of: Valine (V) and Alanine (A), the indicated position being determined by alignment with SEQ ID NO: 1.

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least one amino acid substitution at position 2322, wherein the amino acid at position 2322 is substituted by a polar amino acid selected from the group consisting of: Threonine (T) and Serine (S), the indicated position being determined by alignment with SEQ ID NO: 1.

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least one amino acid substitution at position 2328, wherein the amino acid at position 2328 is substituted by an aliphatic amino acid selected from the group consisting of: Valine (V) and Glycine (G), the indicated position being determined by alignment with SEQ ID NO: 1. In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least one amino acid substitution at position 2328, wherein the amino acid at position 2328 is substituted by a polar amino acid selected from the group consisting of: Threonine (T) and Serine (S), the indicated position being determined by alignment with SEQ ID NO: 1 .

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least one amino acid substitution at position

2334, wherein the amino acid at position 2334 is substituted by an aliphatic amino acid selected from the group consisting of: Methionine (M), Isoleucine (I) and Alanine (A), the indicated position being determined by alignment with SEQ I D NO: 1 .

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least one amino acid substitution at position

2335, wherein the amino acid at position 2335 is substituted by an aliphatic amino acid selected from the group consisting of: Glycine (G) and Alanine (A), the indicated position being determined by alignment with SEQ ID NO: 1 .

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least one amino acid substitution at position 2335, wherein the amino acid at position 2335 is substituted by a polar amino acid selected from the group consisting of: Threonine (T) and Serine (S), the indicated position being determined by alignment with SEQ ID NO: 1 .

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least two amino acid substitutions at positions 2334 and 2335, the indicated position being determined by alignment with SEQ I D NO: 1 . In various embodiments, the amino acid at position 2334 is substituted by an amino acid selected from the group consisting of: Methionine (M), Isoleucine (I) and Alanine (A), and preferably by a Methionine (M). In some embodiments, the amino acid at position 2335 is substituted by an amino acid selected from the group consisting of: Glycine (G), Alanine (A), Threonine (T) and Serine (S), and preferably by a Glycine (G).

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least one amino acid substitution at position 2384, wherein the amino acid at position 2384 is substituted by an Alanine (A), the indicated position being determined by alignment with SEQ ID NO: 1 .

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least one amino acid substitution at position 2384, wherein the amino acid at position 2384 is substituted by a polar amino acid selected from the group consisting of: Asparagine (N), Serine (S) and Threonine (T), the indicated position being determined by alignment with SEQ ID NO: 1

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least one amino acid substitution at position 2387, wherein the amino acid at position 2387 is substituted by an aromatic amino acid selected from the group consisting of: Phenylalanine (F) and Tryptophan (W), the indicated position being determined by alignment with SEQ ID NO: 1 .

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least one amino acid substitution at position 2387, wherein the amino acid at position 2387 is substituted by an aliphatic amino acid selected from the group consisting of: Alanine (A) and Valine (V), the indicated position being determined by alignment with SEQ ID NO: 1.

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least one amino acid substitution at position 2391 , wherein the amino acid at position 2391 is substituted by an aromatic amino acid selected from the group consisting of: Phenylalanine (F) and Tryptophan (W), the indicated position being determined by alignment with SEQ ID NO: 1 .

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least one amino acid substitution at position 2391 , wherein the amino acid at position 2391 is substituted by an aliphatic amino acid selected from the group consisting of: Alanine (A) and Valine (V), the indicated position being determined by alignment with SEQ ID NO: 1.

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least one amino acid substitution selected from the group consisting of: P to V substitution at position 2322 (P2322V), A to V substitution at position 2328 (A2328V), L to M substitution at position 2334 (L2334M), E to G substitution at position 2335 (E2335G), Q to N at position 2384 (Q2384N), Y to F substitution at position 2387 (Y2387F); and Y to F substitution at position 2391 (Y2391 F), the indicated positions being determined by alignment with SEQ ID NO: 1 .

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises at least one amino acid substitution selected from the group consisting of: P to V substitution at position 2322 (P2322V), A to V substitution at position 2328 (A2328V), L to M substitution at position 2334 (L2334M), E to G substitution at position 2335 (E2335G), and Y to F substitution at position 2387 (Y2387F), the indicated positions being determined by alignment with SEQ ID NO: 1 .In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises a single amino acid substitution selected from the group consisting of: P to V substitution at position 2322 (P2322V), A to V substitution at position 2328 (A2328V), L to M substitution at position 2334 (L2334M), E to G substitution at position 2335 (E2335G), Q to N at position 2384 (Q2384N), Y to F substitution at position 2387 (Y2387F), and Y to F substitution at position 2391 (Y2391 F), the indicated positions being determined by alignment with SEQ ID NO: 1 .

In various embodiments, the mutant DNA polymerase theta (Pol theta) or a functional fragment thereof comprises a double amino acid substitution L2334M and E2335G, the indicated positions being determined by alignment with SEQ ID NO: 1 .

In various embodiments, the mutant DNA polymerase or a functional fragment thereof comprises at least one amino acid substitution selected from the group consisting of: P to V substitution at position 2322 (P2322V), A to V substitution at position 2328 (A2328V), L to M substitution at position 2334 (L2334M), E to G substitution at position 2335 (E2335G), and Y to F substitution at position 2387 (Y2387F), the indicated positions being determined by alignment with SEQ ID NO: 1 The terms "mutant" and "variant" may be used interchangeably and constitute the same meaning and purpose within the present invention. A mutant or a variant means a polypeptide derived from DNA polymerases of the pol theta subfamily, or derived from a functional fragment of such DNA polymerases, and in particular of a human DNA polymerase theta sequence according to the sequence SEQ ID NO: 1 , and comprising at least one substitution, and having DNA polymerase and terminal nucleotidyltransferase activities. The variants can be obtained by various techniques well known in the art.

The term "substitution" means that the amino acid in the particular position has been replaced by another amino acid than that in wild-type (wt) DNA polymerase. Preferably, the term "substitution" refers to replacement of an amino acid residue with another selected from the 20 natural residues of standard amino acids, rare amino acid residues of natural origin (e.g. hydroxyproline, hydroxylysine, allohydroxylysine, 6-N- methylysine, N-ethylglycine, N-methylglycine, N-ethylasparagine, allo-isoleucine, N- methylisoleucine, N-methylvaline, pyroglutamine, aminobutyric acid, ornithine), and unnatural amino acid residues (e.g. norleucine, norvalin and cyclohexyl-alanine). Preferably, the term "substitution" refers to the replacement of an amino acid residue by another selected from the standard 20 amino acid residues. For example, a P to V substitution at position 2322 means the replacement of the proline (P) in position 2322 by a valine, the indicated position being determined by alignment with SEQ ID NO: 1 .

The amino acids residues are represented by the one letter or three letter code according to the following nomenclature: A: Ala, alanine; C: Cys, cysteine; D: Asp, aspartic acid; E: Glu, glutamic acid; F: Phe, phenylalanine; G: Gly, glycine; H: His, histidine; I: He, isoleucine; K: Lys, lysine; L: Leu, leucine; M: Met, methionine; N: Asn, asparagine; P: Pro, proline; Q: Gin, glutamine; R: Arg, arginine; S: Ser, serine; T: Thr, threonine; V: Val, valine; W: Trp, tryptophan; Y: Tyr, tyrosine.

The term "aliphatic amino acid" refers to residues having side chain that contain only carbon or hydrogen atoms and remain inside proteins. Aliphatic amino acids group comprises the residues Glycine (G), Alanine (A), Valine (V), Leucine (L) and Isoleucine (I). Methionine residue can be considered as aliphatic amino acid although its side- chain contains a sulfur atom that is largely non-reactive, so that Methionine effectively substitutes well with the strictly aliphatic amino acids.

The term "polar amino acid" refers to residues having a polar group on their side chain. This allows these residues to form hydrogen bonds and covalent bonds to other substituents that may modify the protein structure. The residues Threonine (T), Serine (S), Cysteine (C), Proline (P), Asparagine (N) and Glutamine (Q) constitute the polar amino acids group. The term "aromatic amino acid" refers to residues containing an aromatic ring. Generally, aromatic ring systems are planar, and electrons are shared over the whole ring structure. The aromatic amino acid group comprises Phenylalanine (F), Tryptophane (W) and Tyrosine (Y).

By "comprises at least one substitution", it is meant that the mutant DNA polymerase has one or more amino acid substitutions as indicated with respect to the amino acid sequence SEQ ID NO: 1 , but may have other modifications, including substitutions, deletions or additions of amino acid residues.

The mutant DNA polymerase can comprise 1 , 2, 3, 4, 5, 6, 7 or all of the mutations listed above. All of these possible combinations are specifically contemplated.

Preferably, the mutant DNA polymerase is at least 1000, 1 100, 1200, 1300, 1400, 1500, 1600, 1700, 1725, 1750, or 1775 amino acids in size. Preferably, the mutant contains at least 1 , 2, 3, 4, 5, or 6 substitutions in the Finger subdomain (residues 2333-2474) or the full-length amino acid sequence of Human pol theta (polO), Mouse pol theta (polO), Zebrafish pol theta (polO), or Fruit fly mus308 pol theta (polO) or in a homologous pol theta (polO).

Preferably, the mutant is contained in the plasmids within the bacteria deposited as CNCM 1-5238 (E.coli A1 -1791_CS13); CNCM I-5239 (E.coli A1 -1791_DW9); CNCM I-5240 (E.coli A1 -1791_MC15); and CNCM 1-5241 (E.coli Δ1 -1791_ΝΜ1 1 ).

Preferably, the mutant contains at least 1 , 2, 3, 4, 5, or 6 substitutions in the Finger subdomain (residues 2333-2474) or the full-length amino acid sequence of Human pol theta (polO), Mouse pol theta (polO), Rat pol theta (polO), Chicken pol theta (polO), Canine pol theta (polO), Zebrafish pol theta (polO), or Fruit fly mus308 pol theta (polO) or in a homologous pol theta (polO).

The term "homologous" refers to sequences that have sequence similarity. The term "sequence similarity", in all its grammatical forms, refers to the degree of identity or correspondence between nucleic acid or amino acid sequences. In the context of the invention, two amino acid sequences are "homologous" when at least 80%, alternatively at least 81 %, alternatively at least 82%, alternatively at least 83%, alternatively at least 84%, alternatively at least 85%, alternatively at least 86%, alternatively at least 87%, alternatively at least 88%, alternatively at least 89% alternatively at least 90%, alternatively at least 91 %, alternatively at least 92% alternatively at least 93%, alternatively at least 94%, alternatively at least 95% alternatively at least 96%, alternatively at least 97%, alternatively at least 98% alternatively at least 99% of the amino acids are identical to the Finger subdomain

(residues 2333-2474) or the full-length amino acid sequence of Human pol theta (polO)

(Gene ID. 10721 ), Mouse pol theta (pole) (Gene ID. 77782), Zebrafish pol theta (polO)

(Gene ID. 566079), or Fruit fly mus308 pol theta (polO) (Gene ID. 41571 ).

Preferably, the mutant DNA polymerase has at least 70%, 75%, 80%, 85%, 90%, 95%, 97%, 98%, or 99% identity with the following amino acid sequence (SEQ ID NO:1 ):

1 mnllrrsgkr rrsesgsdsf sgsggdssas pgflsgsvls pppglgrclk aaaageckpt

61 vpdyerdkll lanwglpkav lekyhsfgvk kmfewgaecl llggvlegkn lvysaptsag

121 ktlvaellil krvlemrkka lfilpfvsva kekkyylgsl fgevgikvdg ymgstspsrh 181 fssldiavct ieranglinr lieenkmdll gmvvvdelhm lgdshrgyll ellltkicyi

241 trksascgad lasslsnavg ivgmsatlpn lelvaswlna elyhtdfrpv pllesvkvgn

301 siydssmklv refepmlgvk gdedhvvslc yeticdnhsv llfcpskkwc ekladiiare

361 fynlhhgaeg lvkpsecppv ilegkellev mdglrrlpsg ldsvlgktvp wgvafhhagl

421 tfeerdiieg afrgglirvl aatstlssgv nlparrviir tpifggrpld iltykgmvgr 481 agrkgvdtvg esilicknse kskgiallgg slkpvrsclg rregeevtgs miraileiiv

541 ggvastsgdm htyaactfla asmkegkggi grngesvglg aieacvmwll enefigstea

601 sdgtegkvyh pthlgsatls sslspadtld ifadlgramk gfvlendlhi lylvtpmfed

661 wttidwyrff clweklptsm krvaelvgve egflarcvkg kvvartergh rgmaihkrff

721 tslvlldlis evplreingk ygcnrggigs lggsaavyag mitvfsnrlg whnmelllsg 781 fgkrltfgig relcdlvrvs llnagrarvl yasgfhtvad laraniveve vilknavpfk

841 sarkavdeee eaveerrnmr tiwvtgrkgl tereaaaliv eearmilggd lvemgvgwnp

901 callhsstcs lthsesevke htfisgtkss ykkltsknks ntifsdsyik hspnivgdln

961 ksrehtssfn cnfgngngeh gtcsifrark rasldinkek pgasgnegkt sdkkvvgtfs

1021 gktkkaplnf nsekmsrsfr swkrrkhlkr srdssplkds gacrihlggg tlsnpslced 1081 pftldekkte frnsgpfakn vslsgkekdn ktsfplgikg ncswnitltn dnfvehivtg

1141 sgsknvtcga tsvvsekgrg vaveaekine vligngskng nvymkhhdih pingylrkgs

1201 hegtstitkg kniiergmpc eavssyinrd snvtinceri klnteenkps hfgalgddis

1261 rtvipsevlp sagafskseg ghenflnisr lgektgtytt nktknnhvsd lglvlcdfed

1321 sfyldtgsek iiggmatena klgakdtnla agimgkslvg gnsmnsfgke chipfpaegh 1381 plgatkidhl dlktvgtmkg ssdshgvdil tpespifhsp illeenglfl kknevsvtds

1441 glnsflggyg tgetvkpvil lipgkrtptg vegeclpvpe tslnmsdsll fdsfsddylv

1501 keglpdmgmk eplpsevtsn hfsdslclge dlikksnvne ngdthggltc sndesiifse

1561 mdsvgmveal dnvdifpvge knhtvvspra lelsdpvlde hhggdgdggd gderaekskl

1621 tgtrgnhsfi wsgasfdlsp glgrildkvs spleneklks mtinfsslnr kntelneege 1681 visnletkgv ggisfssnne vkskiemlen nanhdetssl lprkesnivd dnglipptpi

1741 ptsaskltfp giletpvnpw ktnnvlgpge sylfgspsdi knhdlspgsr ngfkdnspis

1801 dtsfslglsg dglgltpass sseslsiidv asdgnlfgtf ikewrckkrf sislacekir

1861 sltssktati gsrfkgassp geipirddgf pikgcddtlv vglavcwggr dayyfslgke

1921 gkhseisasl vppsldpslt lkdrmwylgs clrkesdkec svviydfigs ykilllscgi 1981 slegsyedpk vacwlldpds geptlhsivt sflphelpll egmetsggig slglnagseh

2041 sgryrasves ilifnsmngl nsllgkenlg dvfrkvemps gyclalleln gigfstaece

2101 sgkhimgakl daietgaygl aghsfsftss ddiaevlfle lklppnremk nggskktlgs

2161 trrgidngrk lrlgrgfsts kdvlnklkal hplpglilew rritnaitkv vfplgrekcl 2221 npflgmeriy pvsqshtatg ritftepniq nvprdfeikm ptlvgespps qavgkgllpm 2281 grgkykkgfs vnprcqaqme eraadrgmpf sismrhafvp fpggsilaad ysqlelrila 2341 hlshdrrliq vlntgadvfr siaaewkmie pesvgddlrq qakqicygii ygmgakslge 2401 qmgikendaa cyidsfksry tginqfmtet vknckrdgfv qtilgrrryl pgikdnnpyr 2461 kahaerqain tivqgsaadi vkiatvniqk qletfhstfk shghregmlq sdqtglsrkr

2521 klqgmfcpir ggffilqlhd ellyevaeed vvqvaqivkn emesavklsv klkvkvkiga 2581 swgelkdfdv.

Mutants, homologues, functional fragments of DNA polymerases of the Pol theta family can be made by routine techniques in the art and screened for activity (i.e., capable of performing non-templated nucleic acid extension) using the assays described herein or other similar assays.

Nucleic acids, vectors and cells

In various embodiments, the invention encompasses nucleic acids comprising a nucleotide sequence encoding the mutant DNA polymerase of the invention or a functional fragment thereof, a vector comprising these nucleic acids, and a host cell comprising these vectors. Nucleic acids include DNA, RNA, and modified nucleic acids.

The term "vector" herein means the vehicle by which a heterologous (e.g., mutant or synthetic) DNA or RNA sequence of can be introduced into a host cell so as to transform it and promote expression of the introduced sequence. Preferably, the vector is a DNA vector. Vectors may include for example, plasmids, phages, and viruses and are discussed in greater detail below. Indeed, any type of plasmid, cosmid, YAC or viral vector may be used to prepare a recombinant nucleic acid construct. For example, viral vectors, such as lentiviruses, retroviruses, herpes viruses, adenoviruses, adeno-associated viruses, vaccinia virus, baculovirus, and other recombinant viruses with desirable cellular tropism. Methods for constructing and using viral vectors are known in the art (see, Miller and Rosman, BioTechniques, 7:980-990, 1992).

For non-vertebrate cells, preferred vectors are the arboviruses, the West Nile virus being particularly preferred, which are arthropod vectors. Other vectors that are known to efficiently be expressed in non-vertebrate cells are the baculoviruses.

For vertebrate cells, lentiviral, AAV, baculoviral and adenoviral vectors are preferred. The vectors suited for expression in mammalian host cells can also be of non viral (e.g. plasmid DNA) origin. Suitable plasmid vectors include, without limitation, pREP4, pCEP4 (Invitrogene), pCI (Promega), pCDM8 and pMT2PC, pVAX and pgWiz. For prokaryote cells, plasmid, bacteriophage and cosmid vectors are preferred. Suitable vectors for use in prokaryote systems include without limitation pBR322 (Gibco BRL), pUC (Gibco BRL), pBluescript (Stratagene), p Poly, pTrc; pET 1 1 d; pIN; and pGEX vectors.

For plant cells, plasmid expression vectors such as Ti plasmids, and virus expression vectors such as Cauliflower mosaic virus (CaMV) and tobacco mosaic virus TMV are preferred.

Expression of recombinant proteins in yeast cells can be done using three types of vectors: integration vectors (Yip), episomal plasmids (YEp), and centromeric plasmids (YCp): Suitable vectors for expression in yeast (e.g. S. cerevisiae) include, but are not limited to pYepSed , pMFa, pJRY88, pYES2 (Invitrogen Corporation, San Diego, Calif.) and pTEF-MF (Dualsystems Biotech Product code: P03303).

A sequence "encoding" an expression product, such as a RNA, polypeptide, protein or enzyme, is a nucleotide sequence that, when expressed, results in the production of that RNA, polypeptide, protein or enzyme; i.e., the nucleotide sequence "encodes" that RNA or it encodes the amino acid sequence for that polypeptide, protein or enzyme.

In the context of the present invention, "host" cells are any cells which can be used for producing recombinant proteins, such as "non-vertebrate" (or invertebrate) cells, vertebrate cells, plant cells, yeast cells, or prokaryote cells. They are preferably non-vertebrate and vertebrate cells.

Non-vertebrate (also known as invertebrate) comprises different phyla, the most famous being the Insect, Arachnida, Crustacea, Mollusca, Annelida, Cirripedia, Radiata, Coelenterata and Infusoria. They are now classified into over 30 phyla, from simple organisms such as sea sponges and flatworms to complex animals such as arthropods and molluscs. In the context of the invention, non-vertebrate cells are preferably insect cells, such as Drosophila or Mosquito cells, more preferably Drosophila S2 cells.

Examples of cells derived from vertebrate organisms that are useful as host cell lines include non-human embryonic stem cells or derivative thereof, for example avian EBX cells; monkey kidney CVI line transformed by SV40 sequences (COS-7, ATCC CRL 1651 ); a human embryonic kidney line (293); baby hamster kidney cells (BHK, ATCC CCL 10); Chinese hamster ovary cells (CHO); mouse Sertoli cells (TM4); monkey kidney cells (CVI, ATCC CCL 70); African green monkey kidney cells (VERO- 76, ATCC CRL-1587); human cervical carcinoma cells (HeLa, ATCC CCL 2); canine kidney cells (MDCK, ATCC CCL 34); buffalo rat liver cells (BRL 3A, ATCC CRL 1442); human lung cells (W138, ATCC CCL 75 ); human liver cells (Hep G2, HB 8065); mouse mammary tumor cells (MMT 060562, ATCC CCL51 ); rat hepatoma cells (HTC, MI.5); YB2/0 (ATCC n° CRL1662); NIH3T3; HEK and TRI cells. In the context of the invention, vertebrate cells are preferably EBX, CHO, YB2/O, COS, HEK, NIH3T3 cells or derivatives thereof.

Plant cells which can be used in the context of the invention are the tobacco cultivars Bright Yellow 2 (BY2) and Nicotiana Tabaccum 1 (NT-1 ).

Yeast cells which can be used in the context of the invention are: Saccharomyces cerevisiae, Schizosaccharomyces pombe, and Hansenula polymorpha, as well as methylotropic yeasts like Pichia pastoris and Pichia methanolica.

Prokaryote cells which can be used in the context of the invention are typically E.Coli bacteria or Bacillus Subtilis bacteria.

In various embodiments, the host cell of the invention is the host cell deposited at the Collection Nationale de Cultures de Microorganismes (CNCM) at the Institut Pasteur, 25, Rue du Docteur Roux, 75724 Paris, FR, on September 14, 2017 under the deposit number CNCM I-5238 (E.coli A1 -1791_CS13).

In various embodiments, the host cell of the invention is the host cell deposited at the Collection Nationale de Cultures de Microorganismes (CNCM) at the Institut Pasteur, 25, Rue du Docteur Roux, 75724 Paris, FR, on September 14, 2017 under the deposit number CNCM I-5239 (E.coli A1 -1791_DW9).

In various embodiments, the host cell of the invention is the host cell deposited at the Collection Nationale de Cultures de Microorganismes (CNCM) at the Institut Pasteur, 25, Rue du Docteur Roux, 75724 Paris, FR, on September 14, 2017 under the deposit number CNCM I-5240 (E.coli A1 -1791_MC15).

In various embodiments, the host cell of the invention is the host cell deposited at the Collection Nationale de Cultures de Microorganismes (CNCM) at the Institut Pasteur, 25, Rue du Docteur Roux, 75724 Paris, FR, on September 14, 2017 under the deposit number CNCM 1-5241 (E.coli Δ1 -1791_ΝΜ1 1 ).

In various embodiments, the invention encompasses the use of the nucleic acids, vectors, and host cells of the invention for producing a mutant DNA polymerase.

Methods for producing mutant DNA polymerases

In various embodiments, the invention encompasses a method for generating a mutant DNA polymerase of the Pol theta subfamily or a functional fragment thereof comprising substituting at least one amino acid in a DNA polymerase of the Pol theta family at a position selected from the group consisting of: 2322, 2328, 2334, 2335, 2384, 2387 and 2391 , the indicated positions being determined by alignment with SEQ ID NO: 1 .

In various embodiments, the invention encompasses a method for generating a mutant DNA polymerase of the Pol theta subfamily comprising substituting at least one amino acid in a DNA polymerase of the Pol theta subfamily, wherein the at least one substitution is selected from the group consisting of: a Proline (P) to an aliphatic amino acid or a polar amino acid substitution at position 2322, an Alanine (A) to an aliphatic amino acid or a polar amino acid substitution at position 2328, a Leucine (L) to an aliphatic amino acid substitution at position 2334, a Glutamic acid (E) to an aliphatic amino acid or a polar amino acid substitution at position 2335, a Glutamine (Q) to an aliphatic amino acid or a polar amino acid substitution at position 2384, a Tyrosine (Y) to an aromatic amino acid or an aliphatic amino acid substitution at position 2387, and a Tyrosine (Y) to an aromatic amino acid or an aliphatic amino acid substitution at position 2391 , the indicated positions being determined by alignment with SEQ ID NO: 1.

In various embodiments, the invention encompasses a method for generating a mutant DNA polymerase of the Pol theta subfamily comprising substituting at least one amino acid at position 2322 in a DNA polymerase of the Pol theta subfamily, wherein the amino acid at position 2322 is substituted by an aliphatic amino acid selected from the group consisting of: Valine (V) and Alanine (A), the indicated position being determined by alignment with SEQ ID NO: 1 .

In various embodiments, the invention encompasses a method for generating a mutant DNA polymerase of the Pol theta subfamily comprising substituting at least one amino acid at position 2322 in a DNA polymerase of the Pol theta subfamily, wherein the amino acid at position 2322 is substituted by a polar amino acid selected from the group consisting of: Threonine (T) and Serine (S), the indicated position being determined by alignment with SEQ ID NO: 1 .

In various embodiments, the invention encompasses a method for generating a mutant DNA polymerase of the Pol theta subfamily comprising substituting at least one amino acid at position 2328 in a DNA polymerase of the Pol theta subfamily, wherein the amino acid at position 2328 is substituted by an aliphatic amino acid selected from the group consisting of: Valine (V) and Glycine (G), the indicated position being determined by alignment with SEQ ID NO: 1 .

In various embodiments, the invention encompasses a method for generating a mutant DNA polymerase of the Pol theta subfamily comprising substituting at least one amino acid at position 2328 in a DNA polymerase of the Pol theta subfamily, wherein the amino acid at position 2328 is substituted by a polar amino acid selected from the group consisting of: Threonine (T) and Serine (S), the indicated position being determined by alignment with SEQ ID NO: 1 .

In various embodiments, the invention encompasses a method for generating a mutant DNA polymerase of the Pol theta subfamily comprising substituting at least one amino acid at position 2334 in a DNA polymerase of the Pol theta subfamily, wherein the amino acid at position 2334 is substituted by an aliphatic amino acid selected from the group consisting of: Methionine (M), Isoleucine (I) and Alanine (A), the indicated position being determined by alignment with SEQ ID NO: 1.

In various embodiments, the invention encompasses a method for generating a mutant DNA polymerase of the Pol theta subfamily comprising substituting at least one amino acid at position 2335 in a DNA polymerase of the Pol theta subfamily, wherein the amino acid at position 2335 is substituted by an aliphatic amino acid selected from the group consisting of: Glycine (G) and Alanine (A), the indicated position being determined by alignment with SEQ ID NO: 1 . In various embodiments, the invention encompasses a method for generating a mutant DNA polymerase of the Pol theta subfamily comprising substituting at least one amino acid at position 2335 in a DNA polymerase of the Pol theta subfamily, wherein the amino acid at position 2335 is substituted by a polar amino acid selected from the group consisting of: Threonine (T) and Serine (S), the indicated position being determined by alignment with SEQ ID NO: 1 .

In various embodiments, the invention encompasses a method for generating a mutant DNA polymerase of the Pol theta subfamily comprising substituting at least two amino acids at positions 2334 and 2335 in a DNA polymerase of the Pol theta subfamily. In various embodiments, the amino acid at position 2334 is substituted by an amino acid selected from the group consisting of: Methionine (M), Isoleucine (I) and Alanine (A), and preferably by a Methionine (M), In some embodiments, the amino acid at position 2335 is substituted by an amino acid selected from the group consisting of: Glycine (G), Alanine (A), Threonine (T) and Serine (S), and preferably by a Glycine (G).

In various embodiments, the invention encompasses a method for generating a mutant DNA polymerase of the Pol theta subfamily comprising substituting at least one amino acid at position 2384 in a DNA polymerase of the Pol theta subfamily, wherein the amino acid at position 2384 is substituted by an Alanine (A), the indicated position being determined by alignment with SEQ ID NO: 1 .

In various embodiments, the invention encompasses a method for generating a mutant DNA polymerase of the Pol theta subfamily comprising substituting at least one amino acid at position 2384 in a DNA polymerase of the Pol theta subfamily, wherein the amino acid at position 2384 is substituted by a polar amino acid selected from the group consisting of: Asparagine (N), Serine (S) and Threonine (T), the indicated position being determined by alignment with SEQ ID NO: 1

In various embodiments, the invention encompasses a method for generating a mutant DNA polymerase of the Pol theta subfamily comprising substituting at least one amino acid at position 2387 in a DNA polymerase of the Pol theta subfamily, wherein the amino acid at position 2387 is substituted by an aromatic amino acid selected from the group consisting of: Phenylalanine (F) and Tryptophan (W), the indicated position being determined by alignment with SEQ ID NO: 1.

In various embodiments, the invention encompasses a method for generating a mutant DNA polymerase of the Pol theta subfamily comprising substituting at least one amino acid at position 2387 in a DNA polymerase of the Pol theta subfamily, wherein the amino acid at position 2387 is substituted by an aliphatic amino acid selected from the group consisting of: Alanine (A) and Valine (V), the indicated position being determined by alignment with SEQ ID NO: 1 .

In various embodiments, the invention encompasses a method for generating a mutant DNA polymerase of the Pol theta subfamily comprising substituting at least one amino acid at position 2391 in a DNA polymerase of the Pol theta subfamily, wherein the amino acid at position 2391 is substituted by an aromatic amino acid selected from the group consisting of: Phenylalanine (F) and Tryptophan (W), the indicated position being determined by alignment with SEQ ID NO: 1.

In various embodiments, the invention encompasses a method for generating a mutant DNA polymerase of the Pol theta subfamily comprising substituting at least one amino acid at position 2391 in a DNA polymerase of the Pol theta subfamily, wherein the amino acid at position 2391 is substituted by an aliphatic amino acid selected from the group consisting of: Alanine (A) and Valine (V), the indicated position being determined by alignment with SEQ ID NO: 1 .

In various embodiments, the invention encompasses a method for generating a mutant DNA polymerase of the Pol theta subfamily comprising substituting at least one amino acid in a DNA polymerase of the Pol theta subfamily, wherein the at least one substitution is selected from the group consisting of: P to V substitution at position 2322 (P2322V), A to V substitution at position 2328 (A2328V), L to M substitution at position 2334 (L2334M), E to G substitution at position 2335 (E2335G), Q to N at position 2384 (Q2384N), Y to F substitution at position 2387 (Y2387F); and Y to F substitution at position 2391 (Y2391 F), the indicated positions being determined by alignment with SEQ ID NO: 1 .

In various embodiments, the invention encompasses a method for generating a mutant DNA polymerase of the Pol theta subfamily comprising substituting at least one amino acid in a DNA polymerase of the Pol theta subfamily, wherein the at least one substitution is selected from the group consisting of: P to V substitution at position 2322 (P2322V), A to V substitution at position 2328 (A2328V), L to M substitution at position 2334 (L2334M), E to G substitution at position 2335 (E2335G), and Y to F substitution at position 2387 (Y2387F), the indicated positions being determined by alignment with SEQ ID NO: 1 .

In various embodiments, the invention encompasses a method for generating a mutant DNA polymerase of the Pol theta subfamily comprising substituting a single amino acid in a DNA polymerase of the Pol theta subfamily, wherein the single amino acid substitution is selected from the group consisting of: P to V substitution at position 2322 (P2322V), A to V substitution at position 2328 (A2328V), L to M substitution at position 2334 (L2334M), E to G substitution at position 2335 (E2335G), Q to N at position 2384 (Q2384N), Y to F substitution at position 2387 (Y2387F), and Y to F substitution at position 2391 (Y2391 F), the indicated positions being determined by alignment with SEQ ID NO: 1 .

In various embodiments, the invention encompasses a method for generating a mutant DNA polymerase of the Pol theta subfamily comprising substituting the double amino acid substitution L2334M and E2335G, the indicated positions being determined by alignment with SEQ ID NO: 1 .

In various embodiments, the invention encompasses a method for generating a mutant DNA polymerase of the Pol theta subfamily comprising substituting at least one amino acid in a DNA polymerase of the Pol theta subfamily, wherein the at least one substitution is selected from the group consisting of: P to V substitution at position 2322 (P2322V), A to V substitution at position 2328 (A2328V), L to M substitution at position 2334 (L2334M), E to G substitution at position 2335 (E2335G), and Y to F substitution at position 2387 (Y2387F); and E to G substitution at position 2335 (E2335G), the indicated positions being determined by alignment with SEQ ID NO: 1.

These substitutions can be made using routine methods known in the art, such as those described in the Examples. Preferably, the substitutions are made relative to a nucleic acid having all or part of the DNA sequence encoding Human pol theta (polO), Mouse pol theta (polO), Zebrafish pol theta (polO), or Fruit fly mus308 pol theta (polO) or a homologous pol theta (polO). Preferably, the DNA polymerase has at least 70%, 75%, 80%, 85%, 90%, 95%, 97%, 98%, 99%, or 100% identity with the amino acid sequence of SEQ ID NO:1 . The nucleic acid can be at least 200, 300, 400, 500, 600, 700, 800, 900, 1000, 2000, 3000, 4000, or 5000 etc. nucleotides in size and may correspond to the Finger subdomain (residues 2333-2474) or the full-length amino acid sequence.

In a preferred embodiment, the mutant is made starting with DNA polymerase having at least 70%, 75%, 80%, 85%, 90%, 95%, 97%, 98%, 99%, or 100% identity with the amino acid sequence of SEQ ID NO:1 and substituting amino acids within this DNA polymerase.

In various embodiments, the invention encompasses a method for producing a mutant DNA polymerase culturing the host cell of the invention under culture conditions allowing expression of the polynucleotide encoding said mutant, and optionally recovering said mutant thus expressed from the medium culture or host cells. Methods for incorporating nucleotides

The term "about" refers to a measurable value such as an amount, a temporal duration, a temperature and the like, and is meant to encompass non-limiting variations of +/- 40% or +/- 20% or +/- 10% or +/- 5% or +/- 1 % or +/- 0.1 % from the specified value, as such variations are appropriate.

In one embodiment, the invention encompasses a method for incorporating nucleotides in a template-free manner comprising incubating the mutant DNA polymerase of the invention, or a functional fragment thereof, with nucleotide triphosphates under conditions that allow nucleotide incorporation in the absence of a template.

In various embodiments, the mutant DNA polymerase or a functional fragment thereof is at a concentration of about 0.5 μΜ to about 50 μΜ, preferably at a concentration of about 1 μΜ to about 30 μΜ, preferably at a concentration of about 2 μΜ to about 10 μΜ, and preferably at a concentration of about 5 μΜ.

In various embodiments, the mutant DNA polymerase or a functional fragment thereof is incubated in presence of at least one divalent metal. In one embodiment, the divalent metal is manganese (Mn ²⁺ ), cobalt (Co ²⁺ ), magnesium (Mg ²⁺ ) or a combination thereof. In some embodiments, the divalent metal is manganese (Mn ), magnesium (Mg ²⁺ ) or a combination thereof. In some embodiments, the divalent metal is at a concentration of about 1 mM to about 50 mM. In some embodiments, the divalent metal is at a concentration of about 5 mM. Similarly, in one embodiment, the invention encompasses the use the mutant DNA polymerase of the invention or a functional fragment thereof for incorporating nucleotides in a template-free manner. Exemplary conditions are set forth in the Examples.

In various embodiments, the mutant DNA polymerase or a functional fragment thereof is incubated in presence of sodium chloride (NaCI) at a concentration of about 50 mM to about 300 mM, and preferably at a concentration of about 150 mM.

In various embodiments, the mutant DNA polymerase or a functional fragment thereof is incubated in presence of 5 mM Mn ²⁺ , 20 mM Tris/HCI pH 8, 10% glycerol, 150 mM NaCI, 0.01 % IGEPAL C6-30, 0.1 mg.ml ^"1 BSA (Bovine Serum Albumine).ln various embodiments, the incubation is at least 1 minute for generating a 20-30 nucleotides fragment.

In various embodiments, the incubation is performed at a temperature of about 25°C to 50°C, preferably at a temperature of about 42°C.

The nucleotides can be natural deoxy-ribonucleotides, natural ribonucleotides, modified nucleotides or any combination of natural nucleotides and modified nucleotides. In some embodiments the deoxy-ribonucleotide is dATP, dGTP, dCTP, dTTP, or dUTP. In some embodiments the ribonucleotide is ATP, GTP, CTP, or UTP. In certain non-limiting embodiments, the modified nucleotide may be :

cy3-dUTP, Digoxigenin-I 1-dUTP, Biotin-16AA-dUTP, Texas Red-5-dCTP, Cyanine 3-AA-UTP, 4-Thio-UTP, Biotin-16-AA-dCTP, Ganciclovir Triphosphate, N6-(6- Azido)hexyl-adenosine-5' -triphosphate, 5- Hydroxymethyl-2'-deoxyuridine-5'- Triphosphate;

2' modified nucleotides: 2'-Fluoro-dNTP: 2'-Fluoro-dUTP, 2'-Fluoro-dATP, 2'- Fluoro-dCTP, 2'-Fluoro-dGTP, 2'-Fluoro-dTTP; 2'-amino -dNTP: 2'-Amino-dATP, 2'- Amino-dTTP, 2'-Amino-dCTP, 2'-Amino-dGTP and 2'-Amino-dUTP; preferably, 2'- Amino-dATP or 2'-Amino-dGTP; 2'-0-methyl-dNTP: 2'-0-methyl-dATP, 2'-0-methyl- dUTP, 2'-0-methyl-dCTP, 2'-0-methyl-dGTP; 2'-N3-dNTP : 2'-azido-2'-dATP, 2'-azido- 2'-dUTP, 2'-azido-2'-dCTP _! 2'-azido-2'-dGTP; 2'-0-methyl-ATP and 2'-0-methyl-CTP;

Sugar modified nucleotides: Ara-ATP (Vidarabine triphosphate), Ara-CTP (Cytarabine triphosphate), and FANA;

Base modified nucleotides: 5-methyl-UTP, Etheno-ATP, 2-Aminopurine, and 5- ethynyl-UTP; and

3'-modified nucleotides: 3'-azido-ddATP, 3'-azido-ddCTP, 3'-0-methyl-ATP, 3'- O-methyl-CTP, 3'-0-(2-nitrobenzyl)-2'-dATP, 3'-0-NH ₂ -dATP, 3'-0-NH ₂ -dTTP, 3'-0- NH ₂ -dCTP and 3'-0-NH ₂ -dGTP and others. Other 3'-modified nucleotides include in particular reversible terminators and irreversible terminators. Such types of terminators are well-known in the art. Reversible terminators include for example 3'-O-azidomethyl nucleotides (Palla et al., RSC ADV., 2014, 4, 49342-) and 3'-(2-nitro- benzyl)nucleotides. Irreversible terminators include for example 3'-O-methyl dNTPs,

Preferred modified nucleotides include: 2'-Fluoro-dUTP, 2'-Fluoro-dATP, 2'- Fluoro-dCTP, 2'-Fluoro-dGTP, 2'-Fluoro-dTTP, 2'-Amino-dATP, 5-methyl-UTP, Ara- ATP (Vidarabine triphosphate), Ara-CTP (Cytarabine triphosphate), 2'-O-methyl-ATP, 2'-O-methyl-CTP, Etheno-ATP, 2-Aminopurine, FANA, and 5-ethynyl-UTP.

In various embodiments, the invention encompasses a method for producing degenerate or random nucleotide sequences comprising incubating the mutant DNA polymerase of the invention or a functional fragment thereof with nucleotide triphosphates under conditions that allow degenerate or random nucleotide incorporation to produce degenerate or random nucleotide sequences. Exemplary conditions are set forth in the Examples.

In various embodiments, the mutant DNA polymerase or a functional fragment thereof is at a concentration of about 0.5 μΜ to about 50 μΜ, preferably at a concentration of about 1 μΜ to about 30 μΜ, preferably at a concentration of about 2 μΜ to about 10 μΜ, and preferably at a concentration of about 5 μΜ.

In various embodiments, the mutant DNA polymerase or a functional fragment thereof is incubated in presence of at least one divalent metal. In one embodiment, the divalent metal is manganese (Mn2+), cobalt (Co2+), magnesium (Mg2+) or a combination thereof. In some embodiments, the divalent metal is manganese (Mn2+), magnesium (Mg2+) or a combination thereof. In some embodiments, the divalent metal is at a concentration of about 1 mM to about 50 mM. In some embodiments, the divalent metal is at a concentration of about 5 mM.

In various embodiments, the mutant DNA polymerase or a functional fragment thereof is incubated in presence of sodium chloride (NaCI) at a concentration of about 50 mM to about 300 mM, and preferably at a concentration of about 150 mM.

In various embodiments, the mutant DNA polymerase or a functional fragment thereof is incubated in presence of 5 mM Mn2+, 20 mM Tris/HCI pH 8, 10% glycerol, 150 mM NaCI, 0.01 % IGEPAL C6-30, 0.1 mg.ml-1 BSA (Bovine Serum Albumine).

In various embodiments, the incubation is at least 1 minute for generating a 20-

30 nucleotides fragment.

In various embodiments, the incubation is performed at a temperature of about 25°C to 50°C, preferably at a temperature of about 42°C.

The nucleotides can be natural deoxy-ribonucleotides, natural ribonucleotides, modified nucleotides or any combination of natural nucleotides and modified nucleotides. In some embodiments the deoxy-ribonucleotide is dATP, dGTP, dCTP, dTTP, or dUTP. In some embodiments the ribonucleotide is ATP, GTP, CTP, or UTP. In certain non-limiting embodiments, the modified nucleotide may be cy3-dUTP, Digoxigenin-I 1-dUTP, Biotin-16AA-dUTP, Texas Red-5-dCTP, Cyanine 3-AA-UTP, 4- Thio-UTP, Biotin-16-AA-dCTP, Ganciclovir Triphosphate, N6-(6-Azido)hexyl- adenosine-5' -triphosphate, 5- Hydroxymethyl-2'-deoxyuridine-5'-Triphosphate, 2'- Fluoro-dUTP, 2'-Fluoro-dATP, 2'-Fluoro-dCTP, 2'-Fluoro-dGTP, 2'-Fluoro-dTTP, 2'- Amino-dATP, 2'-Amino-dTTP, 2'-Amino-dCTP, 2'-Amino-dGTP, 2'-Amino-dUTP; 2'-0- methyl-dUTP, 2'-0-methyl-dGTP, 2'-N3 -dATP, 2'-N3 -dCTP, 2'-N3 -dGTP, 2'-N3 - dTTP, 2'-azido-2'-dATP, 2'-azido-2'-dUTP, 2'-azido-2'-dCTP, 2'-azido-2'-dGTP, 2'-0- methyl-ATP, 2'-0-methyl-CTP; 3'-azido-ddATP, 3'-azido-ddCTP, 3'-0-methyl-ATP, 3'- O-methyl-CTP, 3'-0-(2-nitrobenzyl)-2'-dATP, 3'-0-NH ₂ -dATP, 3'-0-NH ₂ -dTTP, 3'-0- NH ₂ -dCTP, 3'-0-NH ₂ -dGTP, 5-methyl-UTP, Ara-ATP (Vidarabine triphosphate), Ara- CTP (Cytarabine triphosphate), 2'-0-methyl-ATP, 2'-0-methyl-CTP, e-ATP, 2- Aminopurine, FANA, and 5-ethynyl-UTP. Preferred modified nucleotides include: 2'- Fluoro-dUTP, 2'-Fluoro-dATP, 2'-Fluoro-dCTP, 2'-Fluoro-dGTP, 2'-Fluoro-dTTP, 2'- Amino-dATP, 2'-azido-2'-dATP, 2'-azido-2'-dUTP _! 2'-azido-2'-dCTP _! 2'-azido-2'-dGTP _! 5-methyl-UTP, Ara-ATP (Vidarabine triphosphate), Ara-CTP (Cytarabine triphosphate), 2'-0-methyl-ATP, 2'-0-methyl-CTP, e-ATP, 2-Aminopurine, FANA, 5-ethynyl-UTP, and reversible and irreversible 3'-modified nucleotide terminators as defined above. The degenerate or random nucleotides sequences can contain at least 0%, 10%, 20%, 30%, 40% or 50% molar ratio of natural deoxy-ribonucleotides or ribonucleotides.

In some embodiments, a fixed nucleotide sequence can be added to the 3' end of the degenerate or random nucleotide sequences. In some embodiments, the degenerate or random nucleotide sequences can be amplified. In some embodiments, the amplified sequences can be cloned into a vector to generate a library of degenerate or random nucleotide sequences.

Generation of functional nucleic acids libraries

In one embodiment, the invention encompasses use the mutant DNA polymerase of the invention or a functional fragment thereof for generating a functional nucleic acid library, and preferably an aptamer library.

In various embodiments, the invention encompasses a kit for generating a functional nucleic acid library, in particular an aptamer library comprising the mutant DNA polymerase of the invention or a functional fragment thereof. In some embodiments, the kit comprises reagents for degenerate or random nucleotide incorporation. In some embodiments, the kit comprises a vector for generating a library of degenerate or random nucleotide sequences.

Routinely, SELEX procedures comprise a step where the aptamer candidates are amplified before the next selection step. Since the newly synthesized RNAs do not have a fixed region at both ends, this region can be added enzymatically at the 3'end of each RNA fragment (the 5' end contains the constant known primer sequence).

In various embodiments, the use of ligation reaction ⁴⁰ can be used for adding a 3' fixed region. For that purpose, commercial enzymes such as T4 RNA ligase I or RtcB ligase (from New England Biolabs) can be suitable. T4 RNA ligase I has been used in the example according to the commercial protocol. In various embodiments, a fixed region can be added at the 3 'end of each RNA fragment. The process for synthesizing the fixed region can be as described in the patent application W02015/159023 (AU2015248673), which is hereby incorporated by reference.

To prepare a functional nucleic acid library, optionally an aptamer library, the functional nucleic acid candidates can be amplified, for example by PCR, before selection.

In various embodiments, the self-amplification step can be performed by using the mutant DNA polymerase theta of the present invention or a functional fragment thereof. By using the same enzyme, it would be possible to execute the entire SELEX procedure in an all-in-one system.

In various embodiments, the self-amplification step can be performed through one-pot isothermal reaction ⁴¹ by the Norovirus RNA replicase (NV3D _p0 i). The amplified nucleic acids can be inserted (i.e., cloned) into any vectors that could contain circularized DNA or RNA for the construction of a library. In some embodiments, a library of degenerate or random nucleotide sequences can be generated. The library can contain at least 10 ⁵ , 10 ⁶ , 10 ⁷ , 10 ⁸ , 10 ⁹ , 10 ¹⁰ , 10 ¹¹ , 10 ¹² , 10 ¹³ , 10 ¹⁴ , 10 ¹⁵ , or 10 ¹⁶ different sequences.

Kits

In one embodiment, the invention encompasses a kit for performing any of the above described methods, wherein the kit comprises the mutant DNA polymerase of the invention or a functional fragment thereof.

In various embodiments, the kit comprises a nucleic acid primer with a 3ΌΗ- end. In some embodiment, the nucleic acid primer is selected from the group consisting of: single-stranded DNA, double-stranded DNA with a 3'-OH single stranded over-hang and single-stranded RNA,

In various embodiments, the kit comprises at least one divalent metal. In some embodiments, the divalent metal is manganese (Mn ²⁺ ), cobalt (Co ²⁺ ), magnesium (Mg ²⁺ ) or a combination thereof. In some embodiments, the divalent metal is manganese (Mn ²⁺ ), magnesium (Mg ²⁺ ) or a combination thereof. In various embodiments, the kit comprises a mixture of nucleotides. In some embodiments, the nucleotides are natural deoxy-ribonucleotides, natural ribonucleotides, modified nucleotides or any combination of natural nucleotides and modified nucleotides. In some embodiments the deoxy-ribonucleotide is dATP, dGTP, dCTP, dATP, or dUTP. In some embodiments the ribonucleotide is ATP, GTP, CTP, or UTP. In certain non-limiting embodiments, the modified nucleotide may be cy3-dUTP, Digoxigenin-I 1-dUTP, Biotin-16AA-dUTP, Texas Red-5-dCTP, Cyanine 3-AA-UTP, 4- Thio-UTP, Biotin-16-AA-dCTP, Ganciclovir Triphosphate, N6-(6-Azido)hexyl- adenosine-5' -triphosphate, 5- Hydroxymethyl-2'-deoxyuridine-5'-Triphosphate, 2'- Fluoro-dUTP, 2'-Fluoro-dATP, 2'-Fluoro-dCTP, 2'-Fluoro-dGTP, 2'-Fluoro-dTTP, 2'- Amino-dATP, 2'-Amino-dTTP, 2'-Amino-dCTP, 2'-Amino-dGTP, 2'-Amino-dUTP; 2'-0- methyl-dUTP, 2'-0-methyl-dGTP, 2'-azido-2'-dATP, 2'-azido-2'-dUTP, 2'-azido-2'- dCTP, 2'-azido-2'-dGTP, 2'-0-methyl-ATP, 2'-0-methyl-CTP; 3'-azido-ddATP, 3'-azido- ddCTP, 3'-0-methyl-ATP, 3'-0-methyl-CTP, 3'-0-(2-nitrobenzyl)-2'-dATP, 3'-0-NH ₂ - dATP, 3'-0-NH ₂ -dTTP, 3'-0-NH ₂ -dCTP, 3'-0-NH ₂ -dGTP, 5-methyl-UTP, Ara-ATP (Vidarabine triphosphate), Ara-CTP (Cytarabine triphosphate), 2'-0-methyl-ATP, 2'-0- methyl-CTP, e-ATP, 2-Aminopurine, FANA, and 5-ethynyl-UTP. Preferred modified nucleotides include: 2'-Fluoro-dUTP, 2'-Fluoro-dATP, 2'-Fluoro-dCTP, 2'-Fluoro-dGTP, 2'-Fluoro-dTTP, 2'-Amino-dATP, 2'-azido-2'-dATP, 2'-azido-2'-dUTP, 2'-azido-2'-dCTP, 2'-azido-2'-dGTP, 5-methyl-UTP, Ara-ATP (Vidarabine triphosphate), Ara-CTP (Cytarabine triphosphate), 2'-0-methyl-ATP, 2'-0-methyl-CTP, e-ATP, 2-Aminopurine, FANA, 5-ethynyl-UTP, and reversible and irreversible 3'-modified nucleotide terminators as defined above.

In various embodiments, the kit comprises a reaction buffer.

In various embodiments, the kit comprises instructions for use.

Applications

The mutant DNA polymerase Pol theta of the present invention, or a functional fragment thereof, may be used in a wide variety of protocols and technologies. For example, in certain embodiment, the mutant DNA polymerase Pol theta of the present invention, or a functional fragment thereof, is used in the fields of molecular biology, genomics, transcriptomics, epigenetics, nucleic acid synthesis, nucleic acid sequencing, and the like. The mutant DNA polymerase Pol theta of the present invention, or a functional fragment thereof, can be used in many technology platforms, including but not limited to microarray, bead, and flow cytometry, and will useful in numerous applications, such as genomic research, drug target validation, drug discovery, diagnostic biomarker identification and therapeutic assessment.

EXAMPLES

Materials and reagents

All chemicals and reagents were purchased from Sigma Aldrich (Saint-Quentin

Fallavier, France) or Thermo Fisher scientifics (Courtaboeuf, France) and were of the highest purity. The commercial enzymes T4 Polynucleotide kinase, T4 DNA ligase and T4 RNA ligase 1 were obtained from New England Biolabs (NEB). The enzymes used for the nucleosides digestion, Benzonase® nuclease, Phosphodiesterase I from Crotalus adamenteus venom and alkaline Phosphatase from bovine intestinal mucosa were purchased from Sigma Aldrich.

Nucleotides analogs

Commercial nucleotides

The following nucleotides were purchased from Jena Bioscience: 5-ethynyl-

UTP, 2-Aminopurine-riboside-5'-triphosphate, 2'-0-methyl-CTP, 2'-0-methyl-ATP, ara- CTP, ara-ATP, epsilon (ε)-ΑΤΡ.

The following ones were purchased from Trilink Biotechnologies: 3'- Deoxynucleotide set (3'-dATP, 3'-dCTP, 3'-dGTP, 3'-Deoxy-5-methyl-UTP, 3'-dUTP), 5-methyl-UTP, ATP, CTP, UTP, GTP, 2-Fluoro-dATP, 2-Fluoro-dCTP 2'-Fluoro-dGTP, 2'-Fluoro-dTTP, 2'-Fluoro-dUTP, 2'-amino-dATP, 2'-amino-dUTP, 2'-amino-dCTP, 2'- amino-dCTP, 2'-amino-dGTP, 2'-0-methyl-dATP, 2'-0-methyl-dUTP, 2'-0-methyl- dCTP, 2'-0-methyl-dGTP, 2'-Azido-dATP, 2'-Azido-dUTP, 2'-Azido-dCTP, 2'-Azido- dGTP. Custom synthetic nucleotides

FANA nucleotide was synthesized by M. Hollenstein from Institut Pasteur, Unite de Chimie Bioorganique des acides nucleiques, CNRS UMR 3523. Protein purification

WT Pol theta (Pol Θ) (residues 1792-2590) was expressed from the pSUM03 ²⁶ construct (from S. Doublie & S. Wallace, Addgene plasmid #78462) in BL21 CodonPlus (DE3) RIPL cells (Agilent technologies). The expression was carried out by autoinduction in Terrific broth EZMix™ supplemented with oc-lactose (2 g/L), D-glucose (0.5 g/L), glycerol (8 ml.L ^"1 ), 100 μg.mL ^"1 of ampicillin and 50 μg.mL ^"1 of chloramphenicol. 6 L of autoinducing medium were inoculated (starting OD ₆ oo, 0.05) and the culture was grown for 60h at 20°C, with saturated cultures reaching a final OD ₆ oo between 5 and 8. The following steps were performed at 4°C. Cells were harvested and resuspended at a ratio of 2.5-3 ml per gram of cell pellet in lysis buffer (50 mM HEPES pH 7.4, 300 mM NaCI, 10% glycerol, 1 mM TCEP, 5 mM Imidazole, 1 ,5% (v/v) IGEPAL C6-30, 5 mM CaCI ₂ , PIERCE™ EDTA-free protease inhibitor tablets and Benzonase® nuclease 500U. Cell lysis was performed by using French press Cell-Disruptor at 20,000 psi. Following clarification by ultracentrifugation at 17,000 rpm for 1 h, two steps of column purification were performed. The supernatant was applied to a Ni-NTA resin through a HisTrap HP column (GE Healthcare Life sciences) which was equilibrated with buffer A (50 mM HEPES pH 7.4, 300 mM NaCI, 20 mM imidazole, 0.005% (v/v) IGEPAL C6-30, 1 mM TCEP, 10% (v/v) glycerol), and eluted with a gradient to 500 mM of imidazole with buffer B (50 mM HEPES pH 7.4, 300 mM NaCI, 500 mM Imidazole, 0.005%(v/v) IGEPAL C6-30, 1 mM TCEP, 10% (v/v) glycerol). Fractions from Ni-NTA chromatography containing pol Θ were then applied to Heparin affinity chromatography after a two-fold dilution of the NaCI content with diluting buffer C (50 mM HEPES pH 7.4, 0.005% (v/v) IGEPAL C6-30, 1 mM TCEP, 10% (v/v) glycerol). The HiTrap Heparin column (GE Healthcare Life sciences) was equilibrated with buffer D (50 mM HEPES pH 7.4, 50 mM NaCI, 0.005% (v/v) IGEPAL C6-30, 1 mM TCEP, and 10% (v/v) glycerol) and eluted with a gradient to 2 M of NaCI with buffer E (50 mM HEPES pH 7.4, 2 M NaCI, 0.005% (v/v) IGEPAL C6-30, 1 mM TCEP, and 10% (v/v) glycerol). The protein fraction was then concentrated and frozen rapidly in a liquid nitrogen bath prior to storage at -80°C.

Generation of mutants

Variant pol theta (pol Θ) constructs were generated by site-directed mutagenesis by using the Quick-Change II XL kit (Agilent technologies) and were purified following the previously described protocol. The oligonucleotides used for the mutagenesis are listed in the supplementary Table II. Oligonucleotides

Oligonucleotides were purchased from Eurogentec with RP-HPLC purity and dissolved in Nuclease-free water. Concentrations were measured by UV absorbance using the absorption coefficient ε at 260 nm provided by Eurogentec. ssDNA primer radiolabelling

Oligonucleotides were labelled as follows: 40 μΜ of ssDNA primer (14-mer) were incubated with [γ- ³² Ρ]ΑΤΡ (Perkin Elmer, 3000 Ci.mM ^"1 ) and T4 polynucleotide kinase (New England Biolabs) for 1 hr at 37°C in a total volume of 25 μΙ_. The reaction was stopped by heating the T4 polynucleotide kinase at 75°C for 10 min. 25 μΜ of label-free ssDNA primer was added to the mix and heated for 5 min up to 90°C, and slowly cooled to room temperature overnight.

Radioactive nucleotidyltransferase assay

5 μΜ of Pol Θ was incubated with 50 nM of 5' ³² P-labeled ssDNA for 0 to 30 min at 42°C in the presence of 5 mM of MnCI ₂ in a total volume of 10 μΙ_ of activity buffer (20 mM Tris pH 8, 150 mM NaCI, 10% glycerol, 0.01 % IGEPAL C6-30, 0.1 mg.ml ^"1 BSA). The reaction was started by addition of 500 μΜ of canonical or modified NTPs and stopped after 15 min at 42°C by adding 10 mM EDTA and 98% formamide. The products of the reaction were resolved by gel electrophoresis on a 15% acrylamide gel and 8 M urea. The 0.4-mm wide gel was run for 3-4 hr at 40 V/cm and scanned by Storm 860 Molecular Dynamics phosphorimager (GE Healthcare). Non-radioactive nucleotidyltransferase assay

Different nucleotides ratios were tested in order to verify that each canonical ribonucleotide was equally incorporated by the polymerase. 5 μΜ of enzyme was incubated with 500 nM of non-labelled ssDNA primer (or ssDNA, or ssRNA primer labelled with ATT0488 at its 5'-end) and a ratio 1 :1 :1 :1 of the four ribonucleotides (500 μΜ each) or with 500 μΜ of ATP, CTP and GTP, and 5 mM of UTP (1 :1 :1 :10) or with 500 μΜ of ATP, CTP and GTP and 2.5 mM of UTP (1 :1 :1 :5) or 500 μΜ of ATP and GTP and 2.5 mM of CTP and UTP (1 :1 :5:5). Additional mixtures were prepared with ATP/CTP and UTP/GTP (500 μΜ each). The reaction was performed in the same activity buffer in presence of 5 mM of MnCI ₂ in a total volume of 100 μΙ_. Synthetic RNA fragments were cleaned-up and used immediately for HPLC analysis and for RNA sequencing, for aptamer library construction, or stored at -80°C. Hydrolysis of synthetic RNA to nucleosides and HPLC analysis

Synthetic RNAs obtained after non-radioactive ssDNA primer extension were hydrolysed according to previous protocol ²⁷ with slight modification. 5 to 80 μg of RNA were first cleaned-up by using the RNA Clean & concentrator™-5 kit (Zymo Research). The clean-up was carried out in two steps to allow the purification of small RNA fragments of size between 17 and 200 nt and at the same time large RNAs (>200 nt). The purified RNA pool was treated with Benzonase®nuclease (20 U), Phosphatase alkaline (1 U), Phosphodiesterase I (0.05 U) in 50 μΐ of digestion buffer (50 mM Tris- HCI pH 8, 1 mM MgCI ₂ , 0.1 mg.mL ^"1 BSA). The mix was incubated at 37°C for 3h and the digestion was kept up overnight at room temperature to insure a total lysis. Ribonucleosides were then cleaned-up by using a 10,000 MWCO Vivaspin®-500 centrifugal concentrator (Sartorius) and centrifuging for 10 min at 4°C. The filtrates were transferred to a 100 μΙ-ν\Ά\ insert tube to be further analyzed by HPLC. A Kromasil 100-5-C18 (150 x 4.6mm) column (Sigma Aldrich) was used for HPLC analysis and the sample were eluted with a gradient of 0 to 20% of 10 ^"3 M Acetonitrile/TEAAc in 15min. Solutions of 0.1 mM of the four ribonucleosides were injected as standards. TruSeq RNA library preparation and sequencing

We used 100 ng of total synthetic RNA and construct the sequencing libraries using the TruSeq Stranded mRNA LT Kit (lllumina, RS-122-2101 , San Diego, CA) as recommended by the manufacturer, except that the fragmentation step was omitted. All the reagents were added to the reaction but the incubation at 94°C was not performed. The directional libraries were controlled on Bioanalyzer DNA1000 Chips (Agilent Technologies, #5067-1504, Santa Clara, CA) and the concentration determined using the QuBit dsDNA HS kit (Q32854, Thermo Fisher Scientific). They were sequenced on an lllumina Hiseq 2500 sequencer using a HiSeq SR cluster kit v4 cBot HS (lllumina, # GD-401 -4001 ) and a HiSeq SBS kit v4 50 cycles (lllumina, # FC-401 -4002) in order to have around 50 millions single end reads of 65 bases per sample.

Different bases compositions were tested before RNA sequencing. A pool of the four nucleotides at a ratio of 1 :1 :1 :1 (500 μΜ each, samples annotated as TV) or with 500 μΜ of ATP, CTP and GTP, and 5 mM of UTP (ratio of 1 :1 :1 :10, samples annotated as Ίθυ) or with 500 μΜ of ATP, CTP and GTP and 2.5 mM of UTP (ratio of 1 :1 :1 :5, sample annotated as '5U) or 500 μΜ of ATP and GTP and 2.5 mM of CTP and UTP (ratio of 1 :1 :5:5, sample annotated as '5U5C). Statistical analyses of TruSeq reads

A FastQC analysis was ran in order to check the quality (Fret score) of the reads for each condition. Thus, statistical analyses were performed using R software v3.3.2, Shortread software v1 .32.0 and Biostrings software v2.42.1 . The software Cutadapt v1.14 was used to remove the TruSeq adapter sequences from the reads.

Aptamer library construction

The synthetic RNA pool, obtained after non-radioactive nucleotidyltransferase assay, was used to build the aptamer library. The RNA synthesis was stopped by the addition of 10mM EDTA and the RNAs were cleaned-up with the RNA Clean & concentrator™-5 kit (#R1015, Zymo Research) according to the protocol recommended by the manufacturer for 17-200nt RNA fragment purification. T4 RNA ligase I (#M0204L, New England Biolabs) has been used to ligate a 5'-phosphorylated ssRNA fragment labelled with Cy5 and preferably blocked at its 3'-end with ddC to avoid automation (oligonucleotide NgRNA-Cy5) to the newly synthesized RNAs (RNA acceptor labelled with ATT0488 at its 5' end). The reaction mixture comprised 1 -20 pmol (preferably 10-20 pmol) of RNA acceptor and 5-40pmol (preferably 20 pmol)of Cy5- NgRNA, 1 mM ATP, 15-25% (v/v) (preferably 20% (v/v) of PEG8000 and RNAsin 1 U in a total volume of 20μΙ_ of T4 RNA ligase I buffer supplied in the manufacturer kit (New England Biolabs). The mixture has been incubated for 16h or overnight at 16°C.

The products of the reaction were resolved by gel electrophoresis on a 8 % or 15% acrylamide gel and 8 M urea. The 0.4-mm wide gel was run for 3-4 hr at 40 V/cm and scanned by Typhoon imager (GE Healthcare). A double imaging has been performed by using Alexa488 filter, for the detection of the acceptor RNA length and by using Cy5 filter for the detection of the ligation of the Cy5-ligRNA. During SELEX (Systematic Evolution of Ligands by Exponential Enrichment) procedures it is critical to control the amplification step of nucleic acids aptamers that have bound to the molecule target. Indeed, it is important to have constant regions at 5'- and 3'-ends on either side of the randomized central sequence. These regions need to be useful for the reverse transcription reaction in order to obtain the complementary DNA and to perform PCR to enrich the pool for the next cycle. The same regions must also not interact or form undesirable secondary structures and influence the 3D conformation of the future aptamer.

CS13 and DW9 mutant exhibited a valuable ability to synthesize random sequence of RNAs by incorporating canonical or modified nucleotides to the 3'-ends of ssDNA fragments. The resulted pool of RNAs served as starting candidates for aptamer library. For that purpose, a fixed fragment of RNA was added to the end of each synthesized RNA. This fragment can serve as matrix strand to amplify the selected aptamer after each cycle of SELEX. The inventors decided to implement a ligation of the fixed fragment to each synthesized RNA by exploiting T4 RNA ligase I activity. The results indicate that the ligation of these fixed oligonucleotides occurs.

As explained just above, the 5'-end constant region of each synthetic RNA is constituted by the RNA (15-mer) or DNA primer (18-mer) used to initiate the elongation reaction. In a second step, the 3'-end constant region has been added by ligation of an RNA oligonucleotide after primer polymerization. The double fluorescence detection performed in the same gel allowed the observation of both the quality of the primer elongation (green fluorescence) and the efficiency of the ligation reaction (red fluorescence). The figure 13 presents the primer elongation rate of the DW9 mutant by adding natural ribonucleotides at 3'-end of a RNA primer. The lane 1 shows the fluorescence signals of both RNA primer (before elongation) and NgRNA (before ligation) and gave an idea of the quality of the signal detection obtained in the conditions of the assay. The lanes 2 to 6 display the elongation of the primer as a function of the incubation time (5 seconds to 15 minutes) and demonstrate an exponential increase of the length of the RNA with the duration of the incubation. The lanes 7 to 1 1 correspond to the same newly synthesized RNA that have acquired the 15-mer constant region by ligation. The efficiency of the ligation has been evaluated by the presence of both green and red fluorescence, resulting in a yellow colour along the gel and corresponding to a varied RNA fragment size. In addition, some thick band are observed at exactly +15, that corresponds to the ligation of the constant region to the non-elongated RNA primers that have remained in the reaction bulk. Finally, the lane 12 shows the absence of auto-ligation when the NgRNA fragments are in presence of T4 RNA ligase. Indeed, no band is visible except the "+0" that corresponds to the length of the NgRNA itself. This further demonstrates (or confirms) that it is possible to add a constant region to the synthetic RNA pool and that the RNA library can hence be screened or amplified for different applications, in particular for aptamer selection.

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Table I : Ribonucleoside composition of the synthesized RNA after enzymatic digestion

Table II: Oligonucleotides used in this study