INCORPORATION BY REFERENCE

This application claims benefit of US provisional application Serial No. 61/180,755 filed May 22, 2009.

FIELD OF THE INVENTION The present invention relates to a method of maintaining and producing plasmids in a gram negative bacterium without the use of antibiotic selection pressure. Further, the invention relates to the drugless plasmids produced as well as formulations and/or compositions containing the drugless plasmids, and formulations and/or compositions containing the protein or immunogen produced using the drugless plasmids, and to methods of administering the formulations and/or compositions to a host. The invention relates to gram negative bacteria containing the drugless plasmids. BACKGROUND OF THE INVENTION

To date, there is an absence of plasmid DNA vectors that are considered safe, potent, and efficient for medicinal use. The presence of antibiotic resistance genes in the delivered plasmids is one of the drawbacks of modern gene therapy and DNA vaccine applications.

Plasmids are extra-chromosomal DNA molecules that are separate from the chromosomal DNA and are capable of replicating independently of the chromosomal DNA (Lipps G. (editor). (2008). Plasmids: Current Research and Future Trends. Caister Academic Press. ISBN 978-1-904455-35-6). Plasmids usually occur naturally in bacteria, but are sometimes found in eukaryotic organisms. They are considered transferable genetic elements, or replicons, capable of autonomous replication within a suitable host. Plasmids are naked DNA and do not encode genes necessary to encase the genetic material for transfer to a new host. Therefore host-to-host transfer of plasmids requires direct, mechanical transfer by conjugation or changes in host gene expression allowing the intentional uptake of the genetic element by transformation (Lipps G. 2008).

The use of plasmid DNA (pDNA) for gene therapy and vaccination is a novel technology with considerable potential in human and animal health care (Mairhofer, J. et al., Biotechnol. J., 3, 83-89, 2008). In addition, plasmids serve as important tools in genetics and biotechnology labs, where they are commonly used to multiply or express particular genes (Russell, David W.; Sambrook, Joseph (2001), Molecular cloning: a laboratory manual. Cold Spring Harbor, N. Y: Cold Spring Harbor Laboratory). Plasmids provide a mechanism for horizontal gene transfer within a population of microbes and typically provide a selective advantage under a given environmental state. For example, plasmids may carry genes that provide resistance to naturally occurring antibiotics. Alternative markers besides antibiotics also exist. For example, the proteins produced by the plasmid may act as toxins which also provide a selective advantage under a given environmental state. Plasmids also can provide bacteria with an ability to fix elemental nitrogen or to degrade recalcitrant organic compounds which provide an advantage under conditions of nutrient deprivation (Lipps G., 2008).

For selection using antibiotic resistant genes, plasmids are prepared wherein a gene is inserted that generates a protein which makes cells resistant to a particular antibiotic. Next, the plasmids are inserted into bacteria by a process called transformation. Then, the bacteria are exposed to the particular antibiotics. Only bacteria which take up copies of the plasmid survive the antibiotic, since the plasmid makes them resistant.

Genes of interest may be delivered using a plasmid containing an antibiotic -resistant marker. Such a gene is typically inserted at a multiple cloning site (MCS, or polylinker). The antibiotic -resistant genes are expressed and the expressed proteins break down the antibiotics. In this way the antibiotics act as a filter to select only the modified bacteria. Now these bacteria can be grown in large amounts, harvested and lysed (often /using the alkaline lysis method) to isolate the plasmid of interest.

The very success of antibiotics in medicine has now become a problem. Many bacteria, including pathogens of infectious diseases, are already resistant and can no longer be controlled with the particular antibiotic. Antibiotics have been used too frequently in human and animal medicine. Moreover, of even far greater significance is the fact that for a long time they were added to animal feed as a performance enhancer. This practice is now largely outlawed, but the pervasive antibiotics have given a survival advantage to those bacteria that have a corresponding resistance gene. Moreover, resistance genes in bacteria are often located on mobile DNA units, which can be exchanged between different species.

Against this background there is concern that bacteria could absorb marker genes eventually resulting in pathogens, against which antibiotics currently being prescribed are ineffective. There might be a gene transfer to environmental microorganisms, e.g., pathogens (Murphy, D. B., Epstein, S. L., Guidance for Industry: Guidance for human somatic cell therapy and gene therapy, Food and Drug Administration, Rockville 1998). Another safety concern is the possible integration of the antibiotic resistance gene into the human chromosome (Smith, H. A., Klinman, D. M., Curr. Opin. Biotechnol. 12, 299-203, 2001). Further, such genes may have a considerable impact on the plasmid production process, as constitutive expression of these genes imposes an unnecessary metabolic load on the bacterial host cell (Cranenburgh, R. M., et al., Nucleic Acids Res. 2001, 29, e26; Rozkov, A., et al., Enzyme Microb. Technol. 2006, 39, 47-50). Reducing the size of the plasmids by eliminating these genes would lead to improved stability and yield of pDNA obtained by the fermentation process (Smith, M. A., et al., Can. J. Microbiol. 1998, 44, 351-355).

Therefore, there is an absolute need in the art to avoid the use of antibiotic resistance genes in the final (commercial) product (naked DNA vaccine) as there is a potential acceptance risk by the public / consumers following current recommendations from regulatory authorities. The Food and Drug Administration (FDA) and the World Health Organization (WHO) regulate the use of antibiotic resistance markers to assure the quality of DNA vaccines and to prevent infectious diseases. Similarly, the EU Deliberate Release Directive, which has been in effect since 2002, requires "the phasing out of the use of antibiotic-resistance markers in genetically modified organisms which may have a harmful impact on human health or the environment". The drawbacks of traditional markers are becoming apparent even in practical research. For example, there is a need to have an antibiotic-free delivery system for the commercial applications of bactofection technology. Bactofection technology is the delivery of plasmid DNA into eukaryotic cells using invasive bacteria. Moreover, there exists a technical need to reduce unnecessary metabolic burdens during the fermentation process, which will achieve higher ODs and higher yields in DNA plasmid.

Alternative selection strategies have been designed to address concerns regarding dissemination of antibiotic resistance genes to a patient's enteric bacteria including auxotrophy complementation, repressor titration, protein based antidote/poison selection schemes, and the use of RNA based selectable markers (see Williams J. A. et al., Plasmid DNA vaccine vector design: Impact on efficacy, safety and upstream production, Biotechnol Adv (2009), doi:10.1016/j.biotechadv.l009.02.003).

Cranenburgh, R.M. et al. reported the construction of two novel Escherichia coli strains (DHllacdapD and DHllacP2dapD) that facilitate the antibiotic-free selection and stable maintenance of recombinant plasmids in complex media. They contain the essential chromosomal gene, dapD, under the control of the lac operator/promoter (Cranenburgh, R. M., et al., 2001). Unless supplemented with IPTG (which induces expression of dapD) or DAP, these cells lyse, however, when the strains are transformed with a multicopy plasmid containing the lac operator, the operator competitively titrates the Lacl repressor and allows expression of dapD from the lac promoter. Thus transformants can be isolated and propagated simply by their ability to grow on any medium by repressor titration selection. No antibiotic resistance genes or other protein expressing sequences are required on the plasmid, and antibiotics are not necessary for plasmid selection.

Mairhofer et al. recently investigated designing bacterial host strains that serve to select and maintain plasmids without the use of any selection marker or other additional sequence on the plasmid. Several bacterial strains were modified, so that the plasmid' s replicational inhibitor RNA I could suppress the translation of a growth essential gene by RNA-RNA antisense reaction (Mairhofer, J. et al, Biotechnol. J., 3, 83-89, 2008). An essential gene (murA) was modified such that a repressor protein (tetR) would hamper its expression (Mairhofer, J. et al., 2008). Only in the presence of plasmid and, hence, RNA I, was tetR turned down and murA expressed. The authors reported that different commercially available plasmids could be selected by various modified Escherichia coli strains. They further designed a minimalistic plasmid devoid of any selection marker.

Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention. SUMMARY OF THE INVENTION

Antibiotic-free plasmids as well as gram-negative bacteria comprising the antibiotic- free plasmids are provided. Compositions comprising the antibiotic-free plasmids comprising a heterologous gene encoding an immunogen or a protein and compositions comprising the immunogen or the protein expressed using the antibiotic-free plasmids are also provided. The gram-negative bacteria are engineered to contain one or more heterologous polynucleotide in the nonessential region of the bacterial chromosome. The antibiotic-free plasmids comprise a polynucleotide encoding a repressor which regulates the expression of the heterologous polynucleotide on the bacterial chromosome. The antibiotic-free plasmids may further comprise one or more polynucleotide encoding an immunogen or a protein.

Methods of the invention include methods of producing antibiotic-free plasmids and methods for transfer of foreign genes into mammalian cells using the antibiotic-free plasmids. These and other embodiments are disclosed or are obvious from and encompassed by, the following Detailed Description. BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description, given by way of example, and which is not intended to limit the invention to specific embodiments described, may be understood in conjunction with the accompanying figures, incorporated herein by reference, in which: Figure 1 describes the first component of the drugless concept which is the cloning of the sacB gene coding for levansucrase in gram-negative bacteria that leads to the rapid death of the transformed bacteria when plated on a medium containing sucrose.

Figure 2 describes the second and third components of the drugless or antibiotic-free concept which is the immunity region of phage λ, wherein cl codes for cl repressor; cro codes for Cro protein; N codes for the transcription anti-terminator; ell codes for cl activator; cIII codes for the cIII protease inhibitor; OL 1, 2 and 3 and OR 1,2 and 3 are operators; PL and PR are the rightward and leftward promoters; PRE is the promoter for repressor establishment; PRM is the promoter for repressor maintenance.

Figure 3 describes the theoretical overview of the "drugless" concept wherein the cl repressor activity, expressed from the pDLl (DrugLess Plasmid) plasmid, inhibits transcription of the toxic sacB gene product placed under the λ Pr promoter located on the cell host chromosome and wherein the host E. coli cells' viability in the presence of sucrose is ensured by a sufficient expression level of the λ cl repressor protein from the plasmid.

Figure 4 describes the "sucrose sensitivity" of the inactivated system wherein a switch to 42°C in temperature inhibits the binding activity of cl repressor to the λ Pr promoter and renders the host cells sensitive to sucrose.

Figure 5 illustrates the combination of the host strain with the antibiotic-free pDLl plasmid, resulting from the transformation with pDLl, and growth in medium containing sucrose leading to drugless plasmid yield. Figures 6A-6C illustrate the experimental plan. FIG 6A provides an overview; FIG

6B illustrates that the ApurN (or AedA) parental strain is resistant to kanmaycin and expresses constitutively SacB ⁺ conferring its sensitivity to sucrose when the cells are incubated at 30°C-37°C. When the ApurN (or AedA) parental strain are transformed with either the pPB829 or pPB838 plasmids, these cells gain the ability to survive and grow in the presence of sucrose when incubated at 30-37 ⁰ C; FIG 6C illustrates that the ApurN (or AedA) parental strain transformed with either the pPB829 or pPB838 plasmids (cl plasmids) is able to grow in LB medium containing either kanamycin, sucrose or chloramphenicol as well. The switch of temperature from 30-37 ⁰ C to 42 ⁰ C leads to the cell death when plated in the presence of sucrose. Figure 7 describes both plasmids (pPB829 and pPB838) marked with the genes encoding chloramphenicol acetyl transferase (Cat ^R ) and containing the λ cl gene. pB838 is a derivative of pMCS5 in which Chloramphenicol acetyl transferase gene (cat) replaces the ampicillin resistance gene. This plasmid contains the cl gene placed under the control of the weak promoter of Kanamycin gene (Pl). pPB829 is a pVR1012 plasmid derivative containing the cat gene. The plasmids contain the cl gene which is placed under the control of the weak promoter (Pl). When introduced in the ApurN (or AedA) parental E. coli strain, these plasmids will enable the cells to grow in the presence of sucrose. Chloramphenicol allows demonstration of the proof of concept.

Figures 8A-E illustrate host strain engineering. FIGs 8A and 8B illustrate the generation of the λPr::sacB Qkan cassette to be inserted by allelic replacement of edA or purN gene, respectively, into the E. coli chromosome. Figure 8C illustrates the engineering of the full sacB cassette {ApurNΩ λPrr.sacB cat) by PCR and joining PCR used for the allelic replacement of the edA gene into the ApurN 'Ω λPr::sacB Km E. coli host strain for the generation of the double sacB cassette E. coli strain.

Figures 9A, 9B and 9C illustrate the use of PCR to demonstrate host strain engineering, namely the deletion of edA and purN in E. coli. FIG. 9A illustrates in part the two PCR products (respectively 151 and 729 bp) that were purified and used as a template in a second PCR step with the PBl 186 and PBl 189 primers and the Phusion DNA polymerase. FIG. 9B illustrates the PCR check showing that the λPr: :sacB ⁺ Ω.Kan cassette is introduced into the chromosome of E. coli. FIG. 9C illustrates that the λPr: :sacB ⁺ Ω.Kan cassette is correctly inserted at the locus by allelic replacement of either edA and/or purN gene(s). Figure 9D illustrates the use of PCR to demonstrate the dual sacB cassette host strain engineering, namely the deletions of both edA and purN in E. coli by allelic replacement. The PCR check shows that both λPr::sacB ⁺ Ω.Kan and λPr::sacB ⁺ Ω.Cat cassettes are correctly introduced at the specific loci into the chromosome of E. coli, as confirmed by sequencing. As shown in Figure 9D: Lanes 1: BJ5183 wt ; 2: BJ5138 ApurN, 3: BJ5138 AedA; 4: BJ5183 ApurN AedA not diluted lμl; 5: BJ5183 ApurN AedA diluted 1/10; 6: Water, 0 bp (control); A: BJ5183 wt; B: BJ5138 ApurN ; C: BJ5138 AedA; D: BJ5183 ApurN AedA not diluted 3 μl; E: BJ5183 ApurN AedA not diluted lμl; F: BJ5183 ApurN AedA diluted 1/10; G: BJ5183 ApurN AedA diluted 1/50; H: Water, 0 bp (control).

Figure 1OA demonstrates that the host strain (ApurN λPr: :sacB ⁺ Ωkan) is highly sensitive (no growth) to sucrose, that cells transformed either with pPB829 or pPB838 plasmid grew well in the presence of sucrose at 30°C, that sacB gene expression was perfectly repressed by the cl gene product synthesized from each cl plasmid and that the plasmid maintenance was 100% efficient as clearly demonstrated by the control experiment performed in parallel with similar cells growing in the presence of chloramphenicol. Raising the temperature to 42°C caused inactivation of the cl gene product and death of the cells plated on LB agar containing either sucrose or chloramphenicol. The robustness of this plasmid maintenance experiments was performed after two successive passages of cells growing in the presence of sucrose or chloramphenicol. Figures lOB-C demonstrate that the dual sacB cassettes E. coli strain (ApurN λPr::sacB ⁺ Ωkan AedA λPr: : sacB ⁺ ΩCat) is highly sensitive (no growth) in the presence of sucrose when incubated at 3O ⁰ C and 42 ⁰ C while some spontaneous mutants appear with the one sacB cassette E. coli strain plated on sucrose. FIG 1OC also demonstrates the lowest sucrose concentration required for the dual sacB cassettes E. coli strain compared to the one sacB cassette E. coli strain (from 10% to 2% final).

Figure 11 illustrates the stability of the pPB829 and pPB838 plasmids in growing cells.

Figure 12A shows a lack of spontaneous mutation in the λPrr.sacB one sacB cassette at 30°C. Figures 12B-C demonstrate that the presence of two sacB cassettes (rather than one) confers a better robustness to sucrose sensitivity (still optimal at 2% sucrose) at temperatures ranging from 3O ⁰ C to 37 ⁰ C. The presence of one sacB cassette on the E. coli chromosome was less robust at sucrose concentration ranging from 2% to 4% at similar temperatures. As confirmed in this plating assay, the mutation rate to sucrose with the two sacB cassettes E. coli strain was undetectable at the lowest sucrose concentration (2%) when incubated at 37 ⁰ C while the mutation rate with the one sacB cassette E. coli strain was ranging from 3.8 xlO ^"6 to 5 xlO ^"5 . An independent set of experiment showed that the mutation rate in the two sacB cassettes E. coli strain was nearly 5 xlO ^"10 when incubated at 37 ⁰ C in the presence of 2% sucrose.

Figures 13A-B show the experimental procedures to enable the screening of the transformed cells by the chloramphenicol (Cat) marked cl plasmids without the use of antibiotic as selection pressure on agar plates.

Figures 14A-C illustrate the efficacy of the experimental procedure to select transformed cells with the cl plasmid only (pPB829 and pPB838) without the use of antibiotic as selection pressure. Figures 15A-B illustrate the stability and yield of the pPB829 and pPB838 plasmids in the growing E .coli cells which contain one copy of the sacB cassette. Figures 15C-E illustrate the stability and yield of cat-free pPB885 plasmid in both one and two sacB cassettes E. coli strain (ApurN λPr::sacB ⁺ Ωkan and ApurN λPr::sacB ⁺ Ωkan AedA λPr: : sacB ⁺ Ωcat, respectively). Figure 15F illustrates the plasmid transfection efficacy (GFP expression) in CHO cells of the antibiotic-free plasmid (pPB896 /Sucrose as selection pressure) and antibiotic plasmid (pCG105 / Cat as selection pressure). No obvious difference is observed in terms of expressed GFP protein between these two selection pressure (Sucrose versus Cat as antibiotic).

Figure 16 shows the pCRblunt + PBl 184-1185 plasmid map. Figure 17 shows the restriction map and characteristics of pPB828. Figure 18 shows the restriction map and characteristics of pPB829. Figure 19 shows the plasmid map and characteristics of pPB838. Figure 20 shows the plasmid map and characteristics of pPB844.

Figure 21 shows the plasmid map and characteristics of pPB845. Figure 22 shows the plasmid map and characteristics of pPB846. Figure 23 shows the plasmid map and characteristics of pPB847. Figure 24 shows the plasmid map of pPB885. The pPB885 feature map shows: two CDS: cl repressor: 324-1037; AP(R): 2321-2990 (complementary), original location

(complement 1626..2483); two replication origin: pUC origin: 1500-2173 (complementary), original location (complement 805..1478); fl origin: 2991-3301 (original location 2615- 3053).

Figure 25 shows the plasmid map of the pPB896. The pPB896 feature map shows: three CDS: GFP: 1675-2397 (original location 1676...2398); cl repressor: 4089-4802;

Cm(R): 4828-5484 (complementary), complement (950-1606); one intron: intron A: 805- 1625 (original location 805-1625); one eukaryotic promoter: CMV promoter: 1-683; one replication origin: 3133-3640 (original location 2425..2932); one terminator: BGH transcriptional terminator/PolyA: 2405-2951 (original location 1697..2243); one 5' UTR: CMV IE 5' UTR: 684-804 (original location 684..804).

Figure 26 shows the sequence alignment of cl repressor proteins and sequence identity percentage.

Figure 27 shows the sequence alignment of sacB proteins and sequence identity percentage. Figure 28 is a table showing the SEQ ID NO assigned to the polynucleotide and protein. Figure 29 shows the sequence alignment of the polynucleotides encoding cl repressor proteins and sequence identity percentage at the nucleotide level.

Figure 30 shows the sequence alignment of the polynucleotides encoding sacB proteins and sequence identity percentage at the nucleotide level.

Figure 31 shows the sequences for Pl promoter, cl native promoter, λPr promoter, λPr promoter+5' UTR+sacB gene, Pl promoter + cl gene, and cl native promoter + cl gene.

DETAILED DESCRIPTION

It is noted that in this disclosure and particularly in the claims, terms such as "comprises", "comprised", "comprising" and the like can have the meaning attributed to it in U.S. Patent law; e.g., they can mean "includes", "included", "including", and the like; and that terms such as "consisting essentially of and "consists essentially of have the meaning ascribed to them in U.S. Patent law, e.g., they allow for elements not explicitly recited, but exclude elements that are found in the prior art or that affect a basic or novel characteristic of the invention. Unless otherwise explained, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. The singular terms "a", "an", and "the" include plural referents unless context clearly indicates otherwise. Similarly, the word "or" is intended to include "and" unless the context clearly indicate otherwise. By "animal" is intended mammals, birds, and the like. Animal or host includes mammals and human. The animal may be selected from the group consisting of equine (e.g., horse), canine (e.g., dogs, wolves, foxes, coyotes, jackals), feline (e.g., lions, tigers, domestic cats, wild cats, other big cats, and other felines including cheetahs and lynx), ovine (e.g., sheep), bovine (e.g., cattle), porcine (e.g., pig), avian (e.g., chicken, duck, goose, turkey, quail, pheasant, parrot, finches, hawk, crow, ostrich, emu and cassowary), primate (e.g., prosimian, tarsier, monkey, gibbon, ape), and fish. The term "animal" also includes an individual animal in all stages of development, including embryonic and fetal stages.

The terms "protein", "peptide", "polypeptide" and "polypeptide fragment" are used interchangeably herein to refer to polymers of amino acid residues of any length. The polymer can be linear or branched, it may comprise modified amino acids or amino acid analogs, and it may be interrupted by chemical moieties other than amino acids. The terms also encompass an amino acid polymer that has been modified naturally or by intervention; for example disulfide bond formation, glycosylation, lipidation, acetylation, phosphorylation, or any other manipulation or modification, such as conjugation with a labeling or bioactive component.

The term "nucleic acid" or "polynucleotide" is used interchangeably and refers to RNA or DNA that is linear or branched, single or double stranded, or a hybrid thereof. The term also encompasses RNA/DNA hybrids. The following are non-limiting examples of polynucleotides: a gene or gene fragment, exons, introns, mRNA, tRNA, rRNA, ribozymes, cDNA, recombinant polynucleotides, branched polynucleotides, plasmids, vectors, isolated DNA of any sequence, isolated RNA of any sequence, nucleic acid probes and primers. A polynucleotide may comprise modified nucleotides, such as methylated nucleotides and nucleotide analogs, uracyl, other sugars and linking groups such as fluororibose and thiolate, and nucleotide branches. The sequence of nucleotides may be further modified after polymerization, such as by conjugation, with a labeling component. Other types of modifications included in this definition are caps, substitution of one or more of the naturally occurring nucleotides with an analog, and introduction of means for attaching the polynucleotide to proteins, metal ions, labeling components, other polynucleotides or solid support. The polynucleotides can be obtained by chemical synthesis or derived from a microorganism.

The term "drugless plasmid" or "antibiotic-free plasmid" is used interchangeably and refers to a DNA plasmid which does not contain an antibiotic selection gene. The term "gene" is used broadly to refer to any segment of polynucleotide associated with a biological function. Thus, genes include introns and exons as in genomic sequence, or just the coding sequences as in cDNAs and/or the regulatory sequences required for their expression. For example, gene also refers to a nucleic acid fragment that expresses mRNA or functional RNA, or encodes a specific protein, and which includes regulatory sequences. The subject matter disclosed herein relates to a novel approach to generating bacterial plasmid DNA vectors by circumventing the use of antibiotic resistance genes to produce safer vaccines and immunogenic compositions.

The subject matter disclosed herein demonstrates a new concept for maintaining a high number of plasmid copies within a gram negative bacterium host cell which is based on three components. The first component is a gram negative bacterium host which expresses a toxic product for the bacterium under defined culture conditions wherein the toxic gene is inserted into a non-essential region of the bacterium chromosome. The second component is the presence of a gene on the gram negative bacterium chromosome wherein the gene encodes a toxic product under the control of a constitutive promoter that can be tightly regulated. The third component is the expression of a specific repressor from a plasmid that regulates the promoter operably linked to the toxic gene on the host chromosome.

The gram negative bacteria contemplated in the present invention include, but are not limited to, Avibacterium, Brucella, Escherichia coli, Haemophilus (e.g., Haemophilus suis), Salmonella (e.g., Salmonella enteridis, Salmonella typhimurium, Salmonella infantis), Shigella, Pasteurella, and Rimeirella.

In one embodiment, the toxic gene is a structural sacB gene that encodes levansucrase. In another embodiment, the toxic gene is a structural sacB gene of Bacillus subtilis which encodes the Bacillus subtilis levansucrase. Expression of sacB in its natural environment is harmless to gram positive bacterium but when expressed in gram negative bacterium, it leads to rapid death of the transformed bacterium when they are plated on a medium containing sucrose.

In one aspect, the present invention provides a sacB protein (levansucrase). In another aspect, the present invention provides a sacB protein having a sequence as set forth in SEQ ID NO:4, 60, 62, 64, 66, 68, 70, 72 or 74, and variant or fragment thereof. In yet another aspect, the present invention provides a sacB protein having at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, 96%, 97%, 98% or 99% sequence identity to a polypeptide having a sequence as set forth in SEQ ID NO:4, 60, 62, 64, 66, 68, 70, 72 or 74. In yet another aspect, the present invention provides fragments and variants of the sacB protein identified above (SEQ ID NO:4, 60, 62, 64, 66, 68, 70, 72 or 74) which may readily be prepared by one of skill in the art using well-known molecular biology techniques. Variants are homologous polypeptides having an amino acid sequence at least about 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identity to the polypeptides of the invention, particularly to the amino acid sequence as set forth in SEQ ID NO:4, 60, 62, 64, 66, 68, 70, 72 or 74.

In another aspect, the present invention provides a polynucleotide, such as a sacB gene, encoding a sacB protein (levansucrase), for example, a polynucleotide encoding a sacB protein having a sequence as set forth in SEQ ID NO:4, 60, 62, 64, 66, 68, 70, 72 or 74. In yet another aspect, the present invention provides a polynucleotide encoding a sacB protein having at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, 96%, 97%, 98% or 99% sequence identity to a polypeptide having a sequence as set forth in SEQ ID NO:4, 60, 62, 64, 66, 68, 70, 72 or 74, or a conservative variant, an allelic variant, a homolog or a fragment comprising at least eight or at east ten consecutive amino acids of one of these polypeptides, or a combination of these polypeptides. In another aspect, the present invention provides a polynucleotide having a nucleotide sequence as set forth in SEQ ID NO:3, 61, 63, 65, 67, 69, 71, or 73, or a variant thereof. In yet another aspect, the present invention provides a polynucleotide having at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, 96%, 97%, 98% or 99% sequence identity to one of a polynucleotide having a sequence as set forth in SEQ ID NO:3, 61, 63, 65, 67, 69, 71, or 73, or a variant thereof.

Other toxic genes which may be utilized in the present invention include, but are not limited to, Listeria or Staphylococcus genes. A toxic gene can be a DNA that is expressed as a toxic gene product (a toxic protein or RNA), or can be toxic in and of itself. Examples of such toxic gene products are well known in the art, and include, but are not limited to, restriction endonucleases (e.g., Dpnl), CcdA/CcdB (Maki S. et al. J. MoI. Biol. 256: 473-482, 1996) and genes that kill hosts in the absence of a suppressing function, e.g. kicB. A toxic gene can alternatively be selectable in vitro, e.g., a restriction site. As used herein, the term "homologs" includes orthologs, analogs and paralogs.

Analogs, orthologs, and paralogs of a wild-type polypeptide can differ from the wild-type polypeptide by post-translational modifications, by amino acid sequence differences, or by both. In particular, homologs of the invention will generally exhibit at least 80-85%, 85-90%, 90-95%, or 95%, 96%, 97%, 98% , 99% sequence identity, with all or part of the wild-type polypeptide or polynucleotide sequences, and will exhibit a similar function.

"Variants" is intended to mean substantially similar sequences. For polynucleotides, a variant comprises a deletion and/or addition of one or more nucleotides at one or more sites within the native polynucleotide and/or a substitution of one or more nucleotides at one or more sites in the native polynucleotide. As used herein, a "native" polynucleotide or polypeptide comprises a naturally occurring nucleotide sequence or amino acid sequence, respectively. Variants of a particular polynucleotide of the invention (i.e., the reference polynucleotide) can also be evaluated by comparison of the percent sequence identity between the polypeptide encoded by a variant polynucleotide and the polypeptide encoded by the reference polynucleotide. "Variant" protein is intended to mean a protein derived from the native protein by deletion or addition of one or more amino acids at one or more sites in the native protein and/or substitution of one or more amino acids at one or more sites in the native protein. Variant proteins encompassed by the present invention are biologically active, that is they the ability to elicit an immune response.

Variants include allelic variants. The term "allelic variant" refers to a polynucleotide or a polypeptide containing polymorphisms that lead to changes in the amino acid sequences of a protein and that exist within a natural population (e.g., a virus species or variety). Such natural allelic variations can typically result in 1- 5% variance in a polynucleotide or a polypeptide. Allelic variants can be identified by sequencing the nucleic acid sequence of interest in a number of different species, which can be readily carried out by using hybridization probes to identify the same genetic locus in those species. Any and all such nucleic acid variations and resulting amino acid polymorphisms or variations that are the result of natural allelic variation and that do not alter the functional activity of gene of interest, are intended to be within the scope of the invention.

The term "conservative variation" denotes the replacement of an amino acid residue by another biologically similar residue, or the replacement of a nucleotide in a nucleic acid sequence such that the encoded amino acid residue does not change or is another biologically similar residue. In this regard, particularly preferred substitutions will generally be conservative in nature, as described above.

The polynucleotides of the disclosure include sequences that are degenerate as a result of the genetic code, e.g., optimized codon usage for a specific host. As used herein, "optimized" refers to a polynucleotide that is genetically engineered to increase its expression in a given species. To provide optimized polynucleotides coding for the polypeptides of the present invention, the DNA sequence of the polypeptide can be modified to 1) comprise codons preferred by highly expressed genes in a particular species; 2) comprise an A+T or G+C content in nucleotide base composition to that substantially found in said species; 3) form an initiation sequence of said species; or 4) eliminate sequences that cause destabilization, inappropriate polyadenylation, degradation and termination of RNA, or that form secondary structure hairpins or RNA splice sites. Increased expression of the polypeptide of the present invention, such as a sacB protein or a cl repressor protein, in said species can be achieved by utilizing the distribution frequency of codon usage in eukaryotes and prokaryotes, or in a particular species. The term "frequency of preferred codon usage" refers to the preference exhibited by a specific host cell in usage of nucleotide codons to specify a given amino acid. There are 20 natural amino acids, most of which are specified by more than one codon. Therefore, all degenerate nucleotide sequences are included in the disclosure as long as the amino acid sequence of the polypeptide encoded by the nucleotide sequence is functionally unchanged.

The "identity" with respect to sequences can refer to the number of positions with identical nucleotides or amino acids divided by the number of nucleotides or amino acids in the shorter of the two sequences wherein alignment of the two sequences can be determined in accordance with the Wilbur and Lipman algorithm (Wilbur and Lipman). When RNA sequences are said to be similar, or have a degree of sequence identity or homology with DNA sequences, thymidine (T) in the DNA sequence is considered equal to uracil (U) in the RNA sequence. Thus, RNA sequences are within the scope of the invention and can be derived from DNA sequences, by thymidine (T) in the DNA sequence being considered equal to uracil (U) in RNA sequences. The sequence identity or sequence similarity of two amino acid sequences, or the sequence identity between two nucleotide sequences can be determined using Vector NTI software package (Invitrogen, 1600 Faraday Ave., Carlsbad, CA).

In another embodiment, the promoter which is operably linked to the toxic gene of the present invention is a promoter from λ phage, λ phage is a temperate phage that lives in E. coll. Once the phage is inside its host, it may integrate itself into the host's DNA. In this state, λ is called a prophage and stays resident within the host's genome, without causing much harm to the host. This way, the prophage gets duplicated with every cell division of the host. The DNA of the prophage that is expressed in this state codes for proteins that look out for signs of stress in the host cell. Stress can be a result of starvation, poisons (like antibiotics), and other factors that can damage or destroy the host. When the stress condition is detected, the prophage becomes active again, excises itself from the DNA of the host cell and enters its lytic cycle. The reactivated phage takes apart the host's DNA and reprograms its protein factory to produce new phages in multiple copies. When all resources of the host are depleted from building new phages, the cell is lysed (the cell membrane is broken down), and the new phages are released.

The lambda repressor gene system consists of (from left to right on the chromosome): c/ gene, OR3, OR2, ORl, cro gene, c/ gene codes for the λ repressor ("the cl repressor protein"). The region of the genome that codes for the cl repressor protein is known as the immunity region. The cl repressor protein is both a positive and a negative regulator of gene transcription. It allows the phage λ to stay "latent" on the chromosome of its host bacterium. The lysogenic state of the λ bacteriophage is maintained by binding of regulatory protein cl to the OR (right operator) and OL (left operator) operators from λ Pl and λ Pr promoters, respectively, preventing transcription of the proteins necessary for the lytic stage. Bacterial cells harboring a lysogenic λ phage are immune to further infection by λ phage. The cl repressor protein inhibits the lytic development of any additional infecting phage particles. In one aspect of the embodiment, the promoter comprises a polynucleotide having a sequence as set forth in SEQ ID NO:5. In another aspect, the promoter comprises a polynucleotide having at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, 96%, 97%, 98% or 99% sequence identity to a polynucleotide having a sequence as set forth in SEQ ID NO:5. In one embodiment, the repressor protein of the present invention is a cl repressor protein, such as the cI857 repressor which is temperature sensitive. A λ phage carrying cI857 as a lysogen will grow at temperatures below 39°C but will then induce lytic growth by an increase in temperature. At 3O ⁰ C, the cl repressor protein is active and binds to the right and left operators of the infecting phage. This prevents transcription of any phage proteins and thus prevents lysis. However, at 42 ⁰ C, the cl repressor is inactivated and cannot bind the promoter operators.

In one aspect, the present invention provides a cl repressor protein. In another aspect, the present invention provides a cl repressor protein having a sequence as set forth in SEQ ID NO:2, 44, 46, 48, 50, 52, 54, 56, or 58, and variant or fragment thereof. In yet another aspect, the present invention provides a cl repressor protein having at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, 96%, 97%, 98% or 99% sequence identity to the polypeptide having a sequence as set forth in SEQ ID NO:2, 44, 46, 48, 50, 52, 54, 56, or 58. In yet another aspect, the present invention provides fragments and variants of the cl repressor protein identified above (SEQ ID NO:2, 44, 46, 48, 50, 52, 54, 56, or 58) which may readily be prepared by one of skill in the art using well-known molecular biology techniques. Variants are homologous polypeptides having an amino acid sequence at least about 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99% identity to the polypeptides of the invention, particularly to the amino acid sequence as set forth in SEQ ID NO:2, 44, 46, 48, 50, 52, 54, 56, or 58. In another aspect, the present invention provides a polynucleotide, such as a cl gene, encoding a cl repressor protein, such as a polynucleotide encoding a cl repressor protein having a sequence as set forth in SEQ ID NO:2, 44, 46, 48, 50, 52, 54, 56, or 58. In yet another aspect, the present invention provides a polynucleotide encoding a cl repressor protein having at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, 96%, 97%, 98% or 99% sequence identity to a polypeptide having a sequence as set forth in SEQ ID NO:2, 44, 46, 48, 50, 52, 54, 56, or 58, or a conservative variant, an allelic variant, a homolog or a fragment comprising at least eight or at east ten consecutive amino acids of one of these polypeptides, or a combination of these polypeptides. In another aspect, the present invention provides a polynucleotide having a nucleotide sequence as set forth in SEQ ID NO:1, 45, 47, 49, 51, 53, 55, 57, or 59, or a variant thereof. In yet another aspect, the present invention provides a polynucleotide having at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, 96%, 97%, 98% or 99% sequence identity to a polynucleotide having a sequence as set forth in SEQ ID NO: 1, 45, 47, 49, 51, 53, 55, 57, or 59, or a variant thereof.

In one embodiment, the promoter driving the cl gene is the native cl gene promoter. In another embodiment, the promoter driving the cl gene is the weak promoter of Kanamycin gene (Pl). In another embodiment, the promoter comprises a polynucleotide having a sequence as set forth in SEQ ID NO:29, 83, or 30. In yet another embodiment, the promoter comprises a polynucleotide having at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, 96%, 97%, 98% or 99% sequence identity to a polynucleotide having a sequence as set forth in SEQ ID NO:29, 83, or 30.

Other promoter/repressor pairs which may be utilized in the methods of the present invention include, but are not limited to, a carB promoter controlled by the CarA-CarS repressor-antirepressor pair, growth hormone gene promoters and repressors and Lac repressor (lad) and the Ptrc-2 promoter.

The mutant promoter/repressor pair to serve as the substrate for the mutagenesis may be selected from among such mutants presently available and described in the art. Studies of the binding of mutant repressors to their operators [See, e.g., Nelson and Sauer, Cell, 42:549 (1985); Nelson and Sauer, J. MoI. Biol. 192:27 (1986); and Gussin, et al, Lambda II, (Hendrix, Roberts, Stahl, and Weisberg, eds) Cold Spring Harbor Press, Cold Spring Harbor, N.Y., p. 93-123 (1983)] permit the selection of known mutant promoter and/or mutant repressor sequences which may provide reduced, but not abolished, transcriptional rates. This selected promoter region is incorporated into a heterologous expression plasmid and then altered by conventional site-directed mutagenesis (see, e.g., Morinaga, et al., Biotechnology, 2:636 ,1984). Alternatively, the repressor-encoding sequence is altered in an analogous fashion. The resultant mutant repressor/promoter pair is analyzed for its ability to promote secretion of the heterologous protein by comparison of expression with the wild-type plasmid not having been subjected to the site-directed mutagenesis.

The subject matter disclosed herein provides a drugless or antibiotic-free concept wherein the repressor activity (such as the cl repressor), expressed from the plasmid, inhibits transcription of a toxic gene product (such as the sacB gene product) placed under a λ phage promoter located on the cell host chromosome and wherein the host cells' viability in the presence of a substrate, such as sucrose, is ensured by a sufficient expression level from the plasmid of a repressor protein from the plasmid (such as the λ cl repressor protein). The growth of the host strain containing the antibiotic-free plasmid in the presence of sucrose ensures an efficient system to maintain and produce DNA plasmids. The term plasmid covers any DNA transcription unit comprising a polynucleotide according to the invention and the elements necessary for its in vivo expression in a cell or cells of the desired host or target; and, in this regard, it is noted that a supercoiled or non- supercoiled, circular plasmid, as well as a linear form, are intended to be within the scope of the invention. One embodiment of the present invention relates to a drugless plasmid comprising one or more repressor gene operably linked to one or more promoter. In one aspect, the drugless plasmid comprises a polynucleotide encoding a cl repressor protein. In another aspect, the drugless plasmid comprises a promoter operably linked to the polynucleotide encoding the cl repressor protein. The promoter may be the native cl gene promoter or the weak promoter of Kanamycin gene. In another aspect, the drugless plasmid further comprises a heterologous polynucleotide encoding an immunogen or a protein. In another aspect, the drugless plasmid comprises a promoter operably linked to the heterologous polynucleotide encoding the immunogen or protein. Any suitable promoter known in the art can be employed in the drugless plasmids according to the present invention, including bacterial, yeast, fungal, insect, mammalian, and plant promoters. The promoter may be, but not limited to, the immediate early cytomegalovirus promoter (CMV-IE) of human or murine origin, or optionally having another origin such as the rat or guinea pig, the Super promoter (Ni, M. et al., Plant J. 7, 661-676, 1995). The CMV-IE promoter can comprise the actual promoter part, which may or may not be associated with the enhancer part. Reference can be made to EP-A-260 148, EP-A-323 597, U.S. Patents Nos. 5,168,062, 5,385,839, and

4,968,615, as well as to PCT Application No WO87/03905. Other promoters from gram- negative bacteria are also suitable including, but not limited to, promoters isolated from Avibacterium, Brucella, Escherichia coli, Haemophilus (e.g., Haemophilus suis), Salmonella (e.g., Salmonella enteridis, Salmonella typhimurium, Salmonella infantis), Shigella, Pasteurella, and Rimeirella.

The plasmids may comprise other expression control elements. It is particularly advantageous to incorporate stabilizing sequence, e.g., intron sequence, for example, the first intron of the hCMV-IE (PCT Application No. WO1989/01036), the intron II of the rabbit β- globin gene (van Ooyen et al., 1979). In another embodiment, the plasmids may comprise 3' UTR. The 3' UTR may be, but not limited to, agrobacterium nopaline synthase (Nos) 3' UTR.

As to the polyadenylation signal (polyA) for the plasmids and viral vectors other than poxviruses, use can be made of the poly(A) signal of the bovine growth hormone (BGH) gene (see U.S. 5,122,458), or the poly(A) signal of the rabbit β-globin gene or the poly(A) signal of the S V40 virus.

A "host cell" denotes a prokaryotic or eukaryotic cell that has been genetically altered, or is capable of being genetically altered by administration of an exogenous polynucleotide, such as a recombinant plasmid or vector. When referring to genetically altered cells, the term refers both to the originally altered cell and to the progeny thereof.

Another embodiment of the present invention relates to a gram-negative bacterium host strain comprising a drugless plasmid. One aspect of the embodiment relates to a gram- negative bacterium host train comprising the drugless plasmid and a heterologous polynucleotide inserted in one or more non-essential region of the host chromosome, and wherein the heterologous polynucleotide is operably linked to a promoter which can be tightly regulated by a repressor. In another aspect of the embodiment, the heterologous polynucleotide inserted in the host chromosome is a sacB gene. In another aspect of the embodiment, the sacB gene is operably linked to a promoter, such as the right promoter of the λ phage. It is recognized that the sacB gene and the promoter may be inserted in multiple copies, for example, two or three copies, in the host chromosome. The non-essential region of the host strain may be the deA or purN gene on the E.coli chromosome. The deA and purN genes are nonessential and deletion of the these genes does not affect the growth rate of the bacteria (Kim, J. et al., Biochemistry. 46, 44:12501-12511). In one aspect, one copy of the sacB gene and promoter cassette is inserted in either deA locus ox purN locus by allelic replacement. In another aspect, two copies of the sacB gene and promoter cassette are inserted in either deA locus ox purN locus. In yet another aspect, two copies of the sacB gene and promoter cassette are inserted in the host chromosome, wherein one copy of the sacB gene and promoter cassette is inserted in the deA locus and the other copy is inserted in the purN locus. The sacB gene and promoter cassette refers to the polynucleotide comprising the sacB gene operably linked to a promoter. In one aspect, the sacB gene and promoter cassette comprises a sacB gene and the right promoter of the λ phage. In another aspect, the sacB gene and promoter cassette comprises a polynucleotide having the sequence as set forth in SEQ ID NO:75.

The subject matter disclosed herein further relates to a vaccine or composition comprising the drugless plasmids comprising a heterologous gene encoding an immunogen or a protein, or a vaccine or composition comprising the immunogen or protein expressed using the drugless plasmids. The vaccine or composition may further comprise a pharmaceutical acceptable carrier.

In one embodiment, the immunogen is selected from a feline pathogen such as, but not limited to, feline herpesvirus (FHV), feline calicivirus (FCV), feline leukemia virus (FeLV), feline immunodeficiency virus (FIV), rabies virus, and the like, and combinations thereof. In another embodiment, the immunogen is selected from a canine pathogen including, but not limited to, rabies virus, canine herpesvirus (CHV), canine parvovirus (CPV), canine coronavirus, Leptospira canicola, Leptospira icterohaemorragiae, Leptospira grippotyphosa, Borrelia burgdorferi, Bordetella bronchiseptica and the like, and combinations thereof.

In yet another embodiment, the immunogen is selected from an equine pathogen, such as equine herpesvirus (type 1 or type 4), equine influenza virus, tetanus, west nile virus, Streptococcus equi, Rhodococcus equi and the like or combinations thereof.

In yet another embodiment, the immunogen is selected from a bovine pathogen, such as rabies virus, bovine rotavirus, bovine parainfluenza virus type 3 (bPIV-3), bovine coronavirus, bovine viral diarrhea virus (BVDV), foot and mouth disease virus (FMDV), bovine respiratory syncytial virus (BRSV), Infectious Bovine Rhinotracheitis virus (IBR), Escherichia coli, Pasteurella multocida, Pasteurella haemolytica, salmonella, Cryptosporidium and the like and combinations thereof.

In still another embodiment, the immunogen is selected from a porcine pathogen such as, but not limited to, swine influenza virus (SIV), porcine circovirus type 2 (PCV-2), porcine reproductive respiratory syndrome virus (PRRS), pseudorabies virus (PRV), porcine parvovirus (PPV), FMDV, Mycoplasma hyopneumoniae, Erysipelothrix rhusiopathiae, Pasteurella multocida, Bordetella bronchiseptica, Escherichia coli, Bluetongue virus, African Horse Sickness virus, Rift Valley Fever, Nipah virus and the like, and combinations thereof.

The term "antigen" or "immunogen" is used interchangeably and means a substance that induces a specific immune response in a host animal. The antigen may comprise a recombinant vector containing an insert with immunogenic properties; a piece or fragment of DNA capable of inducing an immune response upon presentation to a host animal; a protein, a polypeptide, a peptide, an epitope, a hapten, or any combination thereof. Alternately, the immunogen or antigen may comprise a toxin or antitoxin.

As used herein, the terms "pharmaceutically acceptable carrier" and "pharmaceutically acceptable vehicle" are interchangeable and refer to a fluid vehicle for containing vaccine antigens that can be injected into a host without adverse effects. Suitable pharmaceutically acceptable carriers known in the art include, but are not limited to, sterile water, saline, glucose, dextrose, or buffered solutions. Carriers may include auxiliary agents including, but not limited to, diluents, stabilizers (i.e., sugars and amino acids), preservatives, wetting agents, emulsifying agents, pH buffering agents, viscosity enhancing additives, colors and the like.

The cationic lipids containing a quaternary ammonium salt which are advantageously but not exclusively suitable for plasmids, are advantageously those having the following formula:

CH ₃

I +

R ₁ - O - CH ₂ - CH-CH ₂ - N - R ₂ - X

OR ₁ CH ₃ in which Rl is a saturated or unsaturated straight-chain aliphatic radical having 12 to

18 carbon atoms, R2 is another aliphatic radical containing 2 or 3 carbon atoms and X is an amine or hydroxyl group, e.g. the DMRIE. In another embodiment the cationic lipid can be associated with a neutral lipid, e.g. the DOPE.

Among these cationic lipids, preference is given to DMRIE (N-(2-hydroxyethyl)-N,N- dimethyl-2,3-bis(tetradecyloxy)-l -propane ammonium; WO96/34109), advantageously associated with a neutral lipid, advantageously DOPE (dioleoyl-phosphatidyl-ethanol amine; Behr, 1994), to form DMRIE-DOPE.

Advantageously, the plasmid mixture with the adjuvant is formed extemporaneously and advantageously contemporaneously with administration of the preparation or shortly before administration of the preparation; for instance, shortly before or prior to administration, the plasmid-adjuvant mixture is formed, advantageously so as to give enough time prior to administration for the mixture to form a complex, e.g. between about 10 and about 60 minutes prior to administration, such as approximately 30 minutes prior to administration.

When DOPE is present, the DMRIE:DOPE molar ratio is advantageously about 95: about 5 to about 5:about 95, more advantageously about 1: about 1, e.g., 1:1.

The DMRIE or DMRIE-DOPE adjuvant: plasmid weight ratio can be between about 50: about 1 and about 1: about 10, such as about 10: about 1 and about 1: about 5, and advantageously about 1: about 1 and about 1: about 2, e.g., 1:1 and 1:2.

In another embodiment, pharmaceutically or veterinarily acceptable carrier, excipient, or vehicle may be a water-in-oil emulsion. Examples of suitable water-in-oil emulsions include oil-based water-in-oil vaccinal emulsions which are stable and fluid at 4 ⁰ C containing: from 6 to 50 v/v % of an antigen-containing aqueous phase, preferably from 12 to 25 v/v %, from 50 to 94 v/v % of an oil phase containing in total or in part a non- metabolizable oil (e.g., mineral oil such as paraffin oil) and/or metabolizable oil (e.g., vegetable oil, or fatty acid, polyol or alcohol esters), from 0.2 to 20 p/v % of surfactants, preferably from 3 to 8 p/v %, the latter being in total or in part, or in a mixture either polyglycerol esters, said polyglycerol esters being preferably polyglycerol (poly)ricinoleates, or polyoxyethylene ricin oils or else hydro genated polyoxyethylene ricin oils. Examples of surfactants that may be used in a water-in-oil emulsion include ethoxylated sorbitan esters (e.g., polyoxyethylene (20) sorbitan monooleate (Tween 80®), available from AppliChem, Inc., Cheshire, CT) and sorbitan esters (e.g., sorbitan monooleate (Span 80®), available from Sigma Aldrich, St. Louis, MO). In addition, with respect to a water-in-oil emulsion, see also US Patent No. 6,919,084, e.g., Example 8 thereof, incorporated herein by reference. In some embodiments, the antigen-containing aqueous phase comprises a saline solution comprising one or more buffering agents. An example of a suitable buffering solution is phosphate buffered saline. In an advantageous embodiment, the water-in-oil emulsion may be a water/oil/water (W /OfW) triple emulsion (see, e.g., U.S. Patent No. 6,358,500, incorporated herein by reference). Examples of other suitable emulsions are described in U.S. Patent No. 7,371,395, incorporated herein by reference. The vaccine or composition can be administered in dosages and by techniques well known to those skilled in the medical or veterinary arts, taking into consideration such factors as the age, sex, weight, species and condition of the recipient animal, and the route of administration. The route of administration can be percutaneous, via mucosal administration (e.g., oral, nasal, anal, vaginal) or via a parenteral route (intradermal, intramuscular, subcutaneous, intravenous, or intraperitoneal). The vaccine or composition can be administered alone, or can be co-administered or sequentially administered with other treatments or therapies. Forms of administration may include suspensions, syrups or elixirs, and preparations for parenteral, subcutaneous, intradermal, intramuscular or intravenous administration (e.g., injectable administration) such as sterile suspensions or emulsions. The vaccine or composition may be administered as a spray or mixed in food and/or water or delivered in admixture with a suitable carrier, diluent, or excipient such as sterile water, physiological saline, glucose, or the like. The compositions can contain auxiliary substances such as wetting or emulsifying agents, pH buffering agents, adjuvants, gelling or viscosity enhancing additives, preservatives, flavoring agents, colors, and the like, depending upon the route of administration and the preparation desired. Standard pharmaceutical texts, such as "Remington's Pharmaceutical Sciences," 1990 may be consulted to prepare suitable preparations, without undue experimentation.

The subject matter disclosed herein further relates to a method of producing a drugless plasmid comprising the steps of 1) engineering a gram negative bacterium host strain comprising a heterologous polynucleotide inserted by allelic replacement in one or more non-essential region of the host chromosome; 2) constructing a DNA plasmid comprising a polynucleotide encoding a cl repressor protein; 3) transforming the bacterium host strain with the DNA plasmid comprising the gene encoding the cl repressor protein; 4) growing the transformed bacterium host stain in the presence of sucrose at a temperature ranging from 30 ⁰ C to 42°C; and 5) recovering the DNA plasmid.

In one aspect of the embodiment, the DNA plasmid further comprises a heterologous polynucleotide encoding an immunogen or a protein, wherein the heterologous polynucleotide is operably linked to a promoter. The promoter may be a promoter functional in prokaryotic or eukaryotic cell, such as a CMV promoter.

The subject matter disclosed herein further relates to a method of producing a protein or an immunogen using the drugless plasmids comprising the steps of 1) engineering a gram negative bacterium host strain comprising a heterologous polynucleotide inserted by allelic replacement in one or more non-essential region of the host chromosome; 2) constructing a DNA plasmid comprising a polynucleotide encoding a cl repressor protein and a gene encoding an immunogen or a protein; 3) transforming the bacterium host strain with the DNA plasmid comprising the polynucleotide encoding the cl repressor protein and the gene encoding the immunogen or protein; 4) growing the transformed bacterium host stain in the presence of sucrose at a temperature ranging from 30 ⁰ C to 42°C; and 5) recovering the immunogen or protein.

In one aspect of the embodiment, the gene encoding an immunogen or a protein is operably linked to a promoter functional in prokaryotic cells. The promoter may be promoter from gram-negative bacteria, including, but not limited to, promoters isolated from Avibacterium, Brucella, Escherichia coli, Haemophilus (e.g., Haemophilus suis), Salmonella (e.g., Salmonella enteridis, Salmonella typhimurium, Salmonella infantis), Shigella, Pasteurella, and Rimeirella.

In one aspect of the embodiment, the heterologous polynucleotide inserted in the host chromosome encodes a sacB protein. In another aspect, the sucrose concentration may range from 0% to about 20%, about 1% to about 20%, about 1% to about 15%, about 1% to about 10%. In yet another aspect, the sucrose contraction may range from about 1% to about 10%, about 1% to about 2%, about 2% to about 3%, about 3% to about 4%, about 4% to about 5%, about 5% to about 6%, about 6% to about 7%, about 7% to about 8%, about 8% to about 9%, or about 9% to about 10%. In yet another aspect, the sucrose contraction may be about 1%, about 2%, about 3%, about 4%, about 5%, about 6%, about 7%, about 8%, about 9%, or about 10%.

In one aspect of the embodiment, the temperature may range from about 30 ⁰ C to about 42°C. In another aspect, the temperature may range from about 30 ⁰ C to about 41 ⁰ C, about 30 ⁰ C to about 39°C, about 30 ⁰ C to about 38°C, about 30 ⁰ C to about 37°C. In yet another aspect, the temperature may be about 30 ⁰ C, about 31°C, about 32°C, about 33°C, about 34°C, about 35°C, about 36°C, about 37°C, about 38°C, about 39°C, about 40 ⁰ C, or about 41°C.

The term recovering includes but is not limited to collecting, extracting, harvesting, or purifying from the culture medium. An "isolated" biological component (such as a polynucleotide, DNA plasmid, protein or organelle) refers to a component that has been substantially separated or purified away from other biological components in the cell of the organism in which the component naturally occurs, for instance, other chromosomal and extra-chromosomal DNA and RNA, proteins, and organells. Polynucleotides (including DNA plasmids) and proteins that have been "isolated" include polynucleotides and proteins purified by standard purification methods, for example, see Russell, David W.; Sambrook, Joseph (2001), Molecular cloning: a laboratory manual. Cold Spring Harbor, N. Y: Cold Spring Harbor Laboratory. The term also embraces polynucleotides and proteins prepared by recombinant technology as well as chemical synthesis.

The term "purified" as used herein does not require absolute purity; rather, it is intended as a relative term. Thus, for example, a purified polypeptide or polynucleotide preparation is one in which the polypeptide or polynucleotide is more enriched than the polypeptide or polynucleotide is in its natural environment. That is the polypeptide or polynucleotide is separated from cellular components. By "substantially purified" is intended that such that at least 60%, at least 70%, at least 80%, at least 90%, at least 95%, or at least 98%, or more of the cellular components or materials have been removed. Likewise, the polypeptide or polynucleotide may be partially purified. By "partially purified" is intended that less than 60% of the cellular components or material is removed.

The present invention further provides for methods for inducing an immune or protective response in an animal, comprising administering to the animal an immunological composition, a vaccine or a composition according to the invention. The immune responses elicited are notably antibody and/or cellular immune responses, and in particular, a gamma- interferon response.

In particular, the present invention provides for methods to immunize against, or to prevent or to reduce the symptoms caused by, infection of an animal with a pathogenic organism (for example, infection by a virus, bacteria, fungus, or protozoan parasite). The method of the present invention is useful in vertebrate animals including, but not limited to, humans, canines (e.g., dogs), felines (e.g., cats); equines (e.g., horses), bovines (e.g., cattle) and porcine animals (e.g., pigs), as well as in avians including, but not limited to, chickens, turkeys, ducks, geese, a quail, a pheasant, parrots, finches, hawks, crows and ratites (ostrich, emu, cassowary, and the like). The method of the present invention is also useful for providing fish DNA vaccines. In a particular aspect of the invention, these methods consist of the vaccination of pregnant females before parturition by administering a vaccine composition made according to the invention. These methods further include the induction of protective antibodies elicited by the vaccination protocol and the transfer of these protective antibodies from vaccinated pregnant females to their offspring. The transfer of such maternal antibodies subsequently protects the offspring from disease.

The dosage of the vaccine composition made according to the present invention will depend on the species, breed, age, size, vaccination history, and health status of the animal to be vaccinated. Other factors like antigen concentration, additional vaccine components, and route of administration (i.e., subcutaneous, intradermal, oral, intramuscular or intravenous administration) will also impact the effective dosage. The dosage of vaccine to administer is easily determinable based on the antigen concentration of the vaccine, the route of administration, and the age and condition of the animal to be vaccinated. Each batch of antigen may be individually calibrated. Alternatively, methodical immunogenicity trials of different dosages, as well as MPD (Minimum Protective Dose) studies and other screening procedures can be used to determine effective dosage for a vaccine composition in accordance with the present invention without undue experimentation. From the examples presented below, it will be readily apparent what approximate dosage and what approximate volume would be appropriate for using the vaccine composition described herein. The critical factor is that the dosage provides at least a partial protective effect against natural infection, as evidenced by a reduction in the mortality and morbidity associated with natural infection. The appropriate volume is likewise easily ascertained by one of ordinary skill in the art. For example, in avian species the volume of a dose may be from about 0.1 ml to about 0.5 ml and, advantageously, from about 0.3 ml to about 0.5 ml. For feline, canine and equine species, the volume of a dose may be from about 0.2 ml to about 3.0 ml, advantageously from about 0.3 ml to about 2.0 ml, and more advantageously, from about 0.5 ml to about 1.0 ml. For bovine and porcine species, the volume of dose may be from about 0.2 ml to about 5.0 ml, advantageously from about 0.3 ml to about 3.0 ml, and more advantageously from 0.5 ml to about 2.0 ml. Repeated vaccinations may be preferable at periodic time intervals to enhance the immune response initially or when a long period of time has elapsed since the last dose. In one embodiment of the present invention, the vaccine composition is administered as a parenteral injection (i.e., subcutaneously, intradermally, or intramuscularly). The composition may be administered as one dose or, in alternate embodiments, administered in repeated doses of from about two to about five doses given at intervals of about two to about six weeks, preferably from about two to about five weeks. However, one of skill in the art will recognize that the number of doses and the time interval between vaccinations depends on a number of factors including, but not limited to, the age of the animal vaccinated; the condition of the animal; the route of immunization; amount of antigen available per dose; and the like. For initial vaccination, the period will generally be longer than a week and preferably will be between about two to about five weeks. For previously vaccinated animals, a booster vaccination, before or during pregnancy, at about an annual interval may be performed. The present invention also contemplates administering a vaccine composition using a needlefree injector such as Pigjet®, Avijet®, Dermojet® or Biojector® (Bioject, Oregon, USA). An person of ordinary skill in the art is able to adjust the specifications of the injector as required with regard to factors such as the species of the animal to be vaccinated; the age and weight of the animal, and the like without undue experimentation. The present invention further relates to a kit comprising a first vial containing an active ingredient such as an immunogen or pharmaceutical composition and, in a second vial, a diluent made according to the present invention. The immunogen may be in a lyophilized form, a dried form or in aqueous solution as described herein.

The invention will now be further described by way of the following non-limiting examples.

EXAMPLES

Example 1: The Antibiotic-Free Plasmid Maintenance Concept

The following example demonstrates a new concept for maintaining a high number of plasmid copies within a gram-negative host cell which is based on three components: 1. a gram-negative bacterium host which expresses a toxic product for the gram- negative bacterium under defined culture conditions wherein the toxic gene is inserted into one or more non essential region of bacterium host chromosome.

2. The presence on the bacterium chromosome of a gene encoding the toxic product under the control of a constitutive promoter that can be tightly regulated; and

3. Expression of a specific repressor from the plasmid that regulates the promoter operably linked to the toxic gene on the host chromosome. .

In this design, it is the presence of the plasmid that controls the multiplication of the gram-negative bacterium host when it is under defined culture conditions (e.g. in the presence of sucrose).

In the first component, the gram negative bacterium is transformed to express the sacB gene coding for levansucrase (see FIG. 1). The second and third components relate to the cl repressor gene product and the Right λ promoter. The cl gene of λ bacteriophage codes for the λ repressor. The region of the genome that codes for the cl repressor protein is known as the immunity region. The immunity region is illustrated in FIG. 2. A plasmid system expressing the cl gene product should inhibit the transcription of the sacB gene placed under the control of the λ promoter. Such a repression should confer resistance to sucrose and allow both the plasmid maintenance and propagation in the host cells (see FIG 1).

FIG. 3 describes the overview of the "drugless" concept wherein the cl repressor activity, expressed from the plasmid, inhibits transcription of the toxic sacB gene product placed under the λ Pr promoter located on the cell host chromosome and wherein the host E. coli cells' viability in the presence of sucrose is ensured by a sufficient expression level of the λ cl repressor protein from the plasmid. The cl repressor binding to its specific promoter is optimal at temperatures between 3O ⁰ C and 37 ⁰ C (see FIG. 3).

In specific case (such as when using the temperature sensitive cI857 repressor), the cl repressor will not bind the promoter at temperatures higher than 4O ⁰ C. Therefore, a switch to 42 ⁰ C inhibits the binding activity of cl repressor to the λ Pr promoter and should render the host cells sensitive to sucrose (more particularly the levan subproducts) as a consequence.

The 42 ⁰ C temperature incubation in the presence of sucrose is a condition ad hoc to validate the functionality of the entire system and constitutes a "negative proof in support of the full functionality of the system (see FIG 4 and FIG 6C). Indeed, the parental cells λPrr.sacB purNΩ.kan transformed with either the pPB829 or pPB838 plasmid growing in the presence of either Cat or Sucrose were not viable when plated on LB sucrose agar upon incubation at

42 ⁰ C. This demonstrates that the viability of the parental cells is dependent on the maintenance of plasmid comprising the cl gene.

Example 2: Generation of the E. coli Host Strain Harboring the λ Promoter /

Levansucrase Gene

Two engineered host strains containing the λPrr.sacBΩkan cassette were prepared wherein the cassette, containing the sacB gene under the control of the lambda promoter (λPr), is marked with Kanamycin and then introduced into the E. coli chromosome by allelic replacement of either edA or purN gene. These genes are not essential and their deletion does not affect the growth rate. The transformed cells became highly sensitive to sucrose.

FIGS 8A-B show that the λPrr.sacB Ω.kan cassette to be inserted by allelic replacement of edA or purN gene, respectively, into the E. coli chromosome is composed of six independent components that were PCR amplified before being fused by joining PCR. FIG 8D shows that the λPrr.sacB Qkan cassette was inserted into the chromosome of

E. coli by allelic replacement of the edA ox purN gene. This cassette insertion was performed by transforming E. coli with a linear DNA template, containing a deleted gene flanked by up- and downstream homologous region of the chromosome locus, and transferred into a recombination-proficient E. coli strain, such as recD, recB recC she A mutants. The E. coli strain for the engineering of the parental strain was the BJ5183 strain from Stratagene (ref# 200154), wherein the genotype was end Al sbcBC recBC galK met thi-l bidl hsdR (Str ^r ). As shown, the initial phenotype of the E. coli wild type was Kan ^s , Cat ^s and Sucrose ^R . Following the transformation of the strain, the phenotype turned Kan ^R , Cat ^s and Sucrose ^s . This latest phenotype was one of the parental strain used for the further experiments dealing with the demonstration of the functionality of the antibiotic-free plasmid.

FIG 8C shows that the λPrr.sacB Ω.cat cassette to be inserted by allelic replacement of edA gene into the Prλ::sacBΩkan E. coli chromosome was composed of six independent components that were PCR amplified before being fused by joining PCR. The goal of the new λPrr.sacB Ω.cat cassette targeting the deletion of the edA gene by allelic exchange was to engineer a double deletion mutant AedA ApurN expressing two sacB cassettes.

FIG 8E shows that when the parental strain was transformed with the cl plasmid (pPB829 or pPB838), the cells (parental strain + cl plasmid [pPB829 or pPB838]) became resistant to kanamycin (Kan ^R ), chloramphenicol (Cat ^R ) and sucrose (Sucrose ^R ). The figure illustrates the coexistence of the engineered host strain (ApurN λPr::sacB ⁺ Ωkan) or (AedA λPr: : sacB ⁺ Ωkan) and drug plasmid to demonstrate proof of concept when the sacB gene was placed under the control of the lambda promoter and the construct was introduced into the chromosome of E. coli by allelic replacement of the edA or purN gene. The transformed cells became highly sensitive to sucrose. When the chloramphenicol marked plasmids (pPB838 or pPB829) comprising the cl repressor was introduced into the ApurN λPr: :sacB ⁺ Ωkan or AedA λPr::sacB ⁺ Ω.kan strains, the transformed cells became resistant to kanamycin, chloramphenicol and sucrose.

The λPromoter (λPr) was amplified with PB 1232 and PB 1233 primers for purN deletion.

PB1232 primer (SEQ ID NO:6):

(CCGAACAACGCGTGGTTATCGACACCGCAAGGGATAAATATCTAACACCG) and PB1233 primer (SEQ ID NO:7): (CAAACTTTTTGATGTTCATATCCATCTGATCCTCTTCAAAAGGCCACCTG) The λPromoter (λPr) was amplified with PB 1234 and PB 1233 primers for edA deletion. PB1234 (SEQ ID NO:8):

(GACGACAAATTTGTAATCAGGCGAGAGCACCGCAAGGGATAAATATCTAACAC CG) The amplification of λPromoter (λPr) (SEQ ID NO:5) was performed using the pLDR8 plasmid (ATCC# 77357) as DNA template. The sacB gene (SEQ ID NO:3) was amplified with the PB 1192 (SEQ ID NO:9) (ATGGATATGAACATCAAAAAGTTTGC) and PB1193 (SEQ ID NO:10)

(AAACAAATAGGGGTTCCGCGCACATTTATTTGTTAACTGTTAATTGTCCTTG) primers using the pNB350 (Merial proprietary property) as DNA template when the joining PCR was used for the engineering of the λPr::sacB Qkan cassette (see FIG 8A-B). For the engineering of the λPr::sacB Ω.cat cassette, the reverse primer PB1320 (SEQ ID NO:80) (GCCGATCAACGTCTCATTTTCGCCGTTAACAGATCTTTATTTGTTAACTGTTAATT GTCCTTG) was used in place of PBl 193. The Ua promoter was amplified with the PBl 194 (SEQ ID NO:11) (ATGTGCGCGGAACCCCTATTTG) and PB1195 (SEQ ID NO:12) (GACGTTTCCCGTTGAATATGGCTCATACTCTTCCTTTTTCAATATTATTGAAGC) primers using the pCMVβ as DNA template. The Kanamycin resistance gene was amplified with the PB1196 (SEQ ID NO:13) (ATGAGCCATATTCAACGGGAAACG) and PB1197 (SEQ ID

NOI M)(GAAAAACGCCAGCGGCAGAGCTGGCGCTTAGAAAAACTCATCGAGCATC AAATG) primers using the plasmid pLL14 (containing the weak promoter for Kanamycin gene, Merial proprietary material) as DNA template. The chloramphenicol (cat) resistance genee placed under the control of its native cat promoter was amplified with the PB 1321 (SEQ ID NO:81) (AGATCTGTTAACGGCGAAAATGAG) and PB1322 (SEQ ID NO:82) (AAAACGCTACAAAAATGCCCGATCCTTTACGCCCCGCCCTGCCACTCATCGC) primers using the pPB829 as DNA template. The 5' flanking region of the purN gene was amplified with PB1237 (SEQ ID NO:15) (TTTGCGGCCGCTGGTGGTGGTCGCCATGTGCGTTAATGACC) and PB 1199 (SEQ ID NO:16) (TATTCGATAACCACGCGTTGTTCGG) using genomic DNA of the E. coli SCSI strain as DNA template. The 3' flanking region of the purN gene was amplified with PB1200 (SEQ ID NO: 17) (GCGCCAGCTCTGCCGCTGGCGTTTTTC) and PB 1238 (SEQ ID NO: 18) (TTTGGATCCGCTGGTGGATATCATCAAGGCAGTAACGCAGAATG) primers using genomic DNA of the E. coli SCSI strain as DNA template. The 5' flanking region of the edA gene was amplified with the PB1235 (SEQ ID NO:19) (TTTGCGGCCGCTGGTGGTTGAGAACCAGGTGATTGAAGCGCC) and PB 1209 (SEQ ID NO:20) (CTCTCGCCTGATTACAAATTTGTCGTC) primers using genomic DNA of SCSI as DNA template. The 3' flanking region of the edA gene was amplified with the following primers PB1210 (SEQ ID NO:21) (AGGATCGGGCATTTTTGTAGCGT) and PB1236 (SEQ ID NO:22) (TTTCTAGAGCTGGTGGCGACTACCGTGAATCCTGGCAACC) primers using plasmid pLDR8 as template DNA.

These six individual PCR products of the λPr::sacB Qkan cassette or the λPr::sacB Ω.cat to be transferred into the E. coli chromosome were fused together following a joining PCR (FIG 8 A-B-C). These six individual amplified fragments were purified with a PCR clean-up kit (Geneclean turbo kit; MP Biomedicals, CA, USA) and a second round PCR was set up with all the 6 fragments and without primers. The PCR condition was as follows: PCR mixture (final 25 μl), 1 μl of each PCR fragment, 2 μl of dNTP (2.5 mM each, 5 μl of 5xPCR buffer, 11.5 μl sterile distilled water, 0.5 μl Phusion DNA polymerase (Finenzymes, Finland); PCR cycles: 98 ⁰ C 30s, (98 ⁰ C 10 s, 6O ⁰ C 30 s, 72 ⁰ C 5 min)* 15 cycles and 72 ⁰ C 10 min. The third round PCR amplification was performed with 1 μl of the purified second round product by using the PB 1235 and PB 1236 primers fox purN (allelic replacement) and the PB 1237 and PB 1238 primers for edA (allelic replacement). The PCR condition was as follows: PCR mixture (final 50 μl), 1 μl of second PCR fragment, 1 μl of dNTP (2.5 mM each, 10 μl of 5xPCR buffer, 37 μl sterile distilled water, 0.25 μl forward primer, 0.25 μl reverse primer, 0.5 μl Phusion DNA polymerase (Finenzymes, Finland); PCR cycles: 98 ⁰ C 30s, (98 ⁰ C 15 s, 6O ⁰ C 30 s, 72 ⁰ C 4 min)* 35 cycles and 72 ⁰ C 10 min. The final PCR-joined amplicons (4007 bp for the purN allelic replacement and 3407 bp for the edA allelic replacement) were checked on agarose gel and purified before transformation into E.coli. Each joined-PCR amplicon was checked by restriction analysis.

The linear DNA fragment {λPrr.sacB Ω.kan) encoding a sacE gene placed under the control of λPr promoter (SEQ ID NO:75) and Kan resistance gene placed under the control of the bla promoter flanked by two long regions homologous to the DNA sequences bordering the target locus {edA or purN) was integrated into the chromosome of E. coli by electroporation. Approximately 300 transformant candidates (ApurN λPrr.sacB Ω.kan or ΔedA λPrr.sacB Ω.kan single mutant) were obtained on LB agar plate containing kanamycin as selection pressure. 20 colonies were randomly picked and purified by streaking on plates containing kanamycin and the λPrr.sacB Ω.kan cassette insertion at edA or purN locus was verified by PCR (see FIG 9B). The PCR using the PBl 196 and PBl 197 was performed to check the Kanamycin insertion in the chromosome. The PCR using PBl 192 and PBl 193 was performed to verify the presence of the sacB gene in the chromosome. The PCR using PB 1204 (SEQ ID NO:23) (GTGGTGCTTATTTCCGGCAACGG) and PB 1205 (SEQ ID NO:24) (CCAGCCACGCGGCGTTTTCGTGC) primers was performed to verify the absence of purN gene in the chromosome. The PCR using PB 1214 (SEQ ID NO:25) (GACCACCGGCCCGGTTGTACCGG) and PB1215 (SEQ ID NO:26)

(CGGACCCGCGATCGCCTGCAGG) was performed to verify the absence of edA gene) (see FIG 9B). The PCR using PB1213 (SEQ ID NO:27)

(GGTGGATGGCGTCCATTTCTGTGC) and PB 1196 primers was performed to verify the right and correct insertion of the cassette at the edA locus. The PCR using the PB1212 (SEQ ID NO:28) (CAAAAGTGTTAAGCGGTAACCTG) and PB1233 primers was performed to verify the left and correct insertion of the cassette at the edA locus. The PCR using PB 1203 and PBl 196 primers was performed to verify the right and correct insertion of the cassette at the purN locus. The PCR using the PB 1202 and PB 1233 primers was performed to verify the left and correct insertion of the cassette at the purN locus (see FIG 9C). All the PCR checks confirmed the integration at the correct loci. Moreover the functionality of sacB gene has also been confirmed, as shown in FIG 10. The engineered parental strain ApurN λPrr.sacB Ω.kan was unable to grow on LB agar plate containing sucrose (FIG 10).

The linear DNA fragment (λPrr.sacB Ω.cat) encoding a sacB gene (placed under the control of λPr promoter) and Cat resistance gene (placed under the control of its natural cat promoter) flanked by two long regions homologous to the DNA sequences bordering the target locus (edA) was integrated into the chromosome of the ApurN λPrr.sacB Ω.kan E. coli strain by electroporation. Approximately 200 transformant candidates (ApurN λPrr.sacB Ω.kan ΔedA λPrr.sacB Ω.cat double mutant) were obtained on LB agar plate containing chloramphenicol as selection pressure. 20 colonies were randomly picked and purified by streaking on chloramphenicol and the λPrr.sacB Ω.cat cassette insertion at the edA locus was verified by PCR (see FIG 9C) using the primer set PB1212 and PB1213 (see FIG 9C). The PCR product was checked by sequencing to validate the identity of the λPrr.sacB Ω.cat.

Host strain engineering is illustrated in FIG. 8D. FIG. 8E illustrates host strain (one sacB cassette [λPr::sacB Ω.kan]) and drugless plasmid coexistence. The initial E. coli genotype is Kan ^s , Cat ^s , Sucrose ^R . Cassette integration into the chromosome of E. coli ApurN PrλrsacBΩKan generated a new genotype, Kan ^R , Cat ^s , Sucrose ^s . Introduction of the plasmid harboring the cl repressor generated the final genotype, Kan ^R , Cat ^R , Sucrose ^R . Example 3: Generation of Plasmids Harboring the cl Repressor Expression

Cassette (plasmids pPB838-pPB844 to pPB847-pPB885-pPB896)

Summary of Plasmid Construction The pPB828-derived plasmids such as pPB829 and pPB844-pPB847 were generated containing the cl ORF of λ phage, under the control of either its own promoter or the weak promoter of the kanamycin gene (Pl).

The pPB838 plamid containing cl gene placed under the control of the weak promoter of Kanamycin gene (Pl) was generated using an intermediate plasmid commercial vector pMCS5 in which chloramphenicol acetyl transferase gene (cat) replaced the ampicillin resistance gene. (FIG 19).

The pPB844 and pPB845 plasmids contain the cl gene (placed under the control of the Pl promoter) cloned into the pPB828 vector which is a pVR1012 derivative (pVR1020 or 1012 plasmid, VICAL Inc.; Luke et al., 1997; Hartikka et al., 1996, see, e.g., U.S. Patent Nos. 5,846,946 and 6,451,769) containing cat gene. The plasmids differ in the orientation of the cl gene into the plasmid (FIG 20-21).

The pPB846 and pPB847 plasmids contain the cl gene (placed under the control of its own (native) promoter SEQ ID NO: 30) cloned into pPB828 vector which is a pVR1012 derivative containing cat gene. The plasmids differ in the orientation of the cl gene in the plasmid (FIG 22-23).

The pPB885 was prepared from the pPB838 plasmid [cl gene placed under the control of the weak promoter of Kanamycin gene (Pl)] in which the chloramphenicol acetyl transferase gene (cat) was removed by Avrll restriction digestion (FIG 24).

The pPB829-derived plasmids such as pPB896 contains cl gene placed under the control of the weak promoter of Kanamycin gene (Pl). The pPB896 plasmid contains the GFP marker gene placed under the control of the eukaryotic CMV promoter (FIG 25).

These plasmids were used as vectors for cloning a heterologous polynucleotide, a gene of interest (transgene) in the drugless system. Each plasmid harbors the antibiotic resistance gene (cat) that can be excised and removed by a unique restriction enzyme to allow the plasmid propagation in the appropriate E. coli sacB ⁺ strain in the presence of sucrose without antibiotic selection pressure (such as chloramphenicol).

The antibiotic resistance gene (cat) on these plasmids was used only for proof of concept. The cat gene is finally removed in the final drugless plasmids (see FIGs 13-15 and Example 4).

The cl gene sequence from plasmid pLDR8 (ATCC Number # 77357) produces a repressor protein cI857 mutant (NCBI: AB248924, Cloning vector pND707,Love, CA. et al., Gene. 17; 176(l-2):49-53; 1996)

A. Construction of the plasmid pPB829 containing the cl repressor ORF placed under the control of the weak promoter Pl

pPB828 plasmid: (pVR1012-based plasmid + Cat gene) (FIG 17) The pPB828-derived plasmids such as pPB829 was generated containing the cl ORF of λ phage, under the weak promoter of the kanamycin gene (Pl) and the Cat gene that is excisable by a unique EcoRI digestion site. A pVR1012-based expression vector was generated containing the chloramphenicol acetyl transferase (cat) gene giving rise to the pPB828 plasmid. The cat gene from pPB627 (Merial proprietary material) was a derivative from pNB335 (Merial proprietary material) which is itself a derivative from initial plasmid pSW23T (cloning plasmid for GenBank Accession No. AY733066).The DNA fragment corresponding to the cat gene (798 bp) (SEQ ID NO:31 for DNA, SEQ ID NO:32 for protein)was obtained by PCR using the primers PB 1184 (SEQ ID NO:33) (GAATTCCGGTCCGGGCGAAAATGAGACGTTGATCGGCS) [Containing an Ecorl site (underlined)] and PB 1185 (SEQ ID NO:34)

(CCTAGGCTGTGTTAATTAAGGCGCGCCGAATTCCGGTCCGTTACGCCCCGCCCTG CCACTCATCGC) [containing an EcoRI site (underlined)], using pNB350 as a template and Phusion™ High-Fidelity DNA polymerase (Finnzymes, 02150 Espoo, Finland). The resulting PCR product (832 bp) was ligated into the pCR blunt Topo vector (Invitrogen, CA, USA) to obtain plasmid pCRblunt/PBl 184-1185 (4351 bp) (FIG 16). The identity of pCRblunt/PBl 184-1185 was confirmed by restriction analysis (PvuII digestion). The DNA fragment corresponding to the Cat gene was obtained by enzyme digestion of pCRblunt/PBl 184-1185 plasmid with EclBόll and EcoRV. The 897 bp fragment was ligated into the pVR1012-based plasmid, previously digested with Mscl and Stul (3303 pb) to generate plasmid pPB828 (FIG 17). The identity of pPB828 was confirmed by restriction analysis {PvuII and Ncol digestion to determine the size of the insert and EcoRI to verify that the gene cat is well removed). pPB829 plasmid: (pPB828 plasmid + Plr.clgene) (FIG 18) The construct containing the cl gene under Pl promoter (weak promoter of Kanamycin resistance gene) was obtained with a fusion PCR. The DNA fragment corresponding to the Pl weak promoter was obtained by PCR using primers PB 1186 (SEQ ID NO:35) (TCATACCAGGCCTAGGTGATACGCCTATTTTTATAGGTTAATG) and PBl 187 (SEQ ID NO:36) (AACACCCCTTGTATTACTGTTTATG), using pLL14 (see Merial patent application US2005/0164946) as a template and Phusion DNA polymerase. The DNA fragment corresponding to the cl gene (SEQ ID NO:1) was obtained by PCR using primers PBl 188 (SEQ ID NO:37) (AATACAAGGGGTGTTATGAGCACAAAAAAGAAACCATTAACAC) [containing a complementary region of Plsequence at 5' end (underlined)] and PBl 189 (SEQ ID NO:38) (CCGGAATTCGGCGCGTCAGCCAAACGTCTCTTCAGGCCACTG) using pLDR8 (ATCC # 77357) as a template and Phusion DNA polymerase. The two PCR products (respectively 151 and 729 bp) were purified and used as template in a second PCR step with the PBl 186 and PBl 189 primers and the Phusion DNA polymerase (Finnzymes, Finland)

(see FIG. 24). The two PCR products (respectively 151 and 729 bp) were purified and ligated with plasmid pPB828 digested with Ascl and Avrll to generate plasmid pPB829 (4970 bp) (FIG 9A).The ligation was set up with the "In-Fusion PCR Cloning Kit" (cat. No. 740590.250) from Clontech (Takara Bio Europe, St-Germain-en-Laye,France). Clones pPB829 were selected following a HindII digestion analysis The integrity of the sequence of cl gene and Pl promoter was confirmed by sequencing the plasmid pPB829. pPB896 plasmid: (pPB829 plasmid + Plr.clgene + GFP) (FIG 25) The GFP marker gene was placed under the control of the eukaryotic CMV promoter (SEQ ID NO:43). The GFP gene was prepared from restriction digestion of the pCG105 plasmid (pPB828-derived plasmid having the GFP gene placed under the control of the eukaryotic CMV promoter, Merial proprietary material) with both Notl/Bglll. This digested GFP gene was cloned into the pPB829 plasmid previously cut by Notll BgIII giving rise to the pPB896 plasmid.

B. Construction of the intermediate plasmid, pPB838, and plasmids pPB844- pPB845, containing the cl gene under the Pl promoter (weak promoter of kanamycin gene) pPB837.1 plasmid: (pMCS5 plasmid + Cat gene)

The DNA fragment corresponding to the Cat gene was obtained by PCR using the primers PB 1182 (SEQ ID NO:39)

(AGATCTGTTAACGGCGAAAATGAGACGTTGATCGGC) [containing a BgIII site

(underlined) at 5' end] and PBl 183 (SEQ ID NO:40) (GTCGACGTTAACTTACGCCCCGCCCTGCCACTCATCGC) [contains a Sail site

(underlined) at 5' end], using pPB791 [derivative of plasmid pNB350] as a template and

Phusion DNA polymerase.

PCR primers were designed based on the cat sequence of pNB350. Three independent

PCR products (779 bp) were pooled and ligated into the pCRII vector to obtain plasmid pCRII + PBl 182-1183 (4734 bp). Clones of pCRII + PBl 182-1183 were selected following a

Sall/Bglll digestion. The pMCS5 plasmid (MoBiTec, Germany) was digested with Sail and

BgIII in order to obtain the DNA fragment A (2950 bp). pCRII + PBl 182-1183 was digested by Sail and BgIII and fragment SaII-BgIII was isolated from agarose gel (797 pb: fragment

B). Fragments A and B were ligated to generate plasmids pPB837.1 (3747 bp).

pPB837.2: (pMCS5 + Cat) without bla promoter Plasmid pPB837.2 was generated which does not contain bla promoter region.

Plasmid pPB837.1 was digested with Nael and Xmnl and fragment Nael-Xmnl of 3300 bp was isolated and purified from agarose gel (fragment A). The fragment A was ligated to obtain the pPB837.2 plasmid (3300bp). Clone pPB837.2 was selected following a BgII digestion.

pPB838 plasmid (pMCS5 + Cat without bla promoter) + Pl::cl (FIG

22) The construct containing the cl gene under Pl promoter (weak promoter of

Kanamycin resistance gene) was obtained with a fusion PCR. The DNA fragment corresponding to the Pl weak promoter was obtained by PCR using primers PB 1186 and PBl 187, the pLL14 as a template and Phusion DNA polymerase. The DNA fragment corresponding to the cl gene was obtained by PCR using primers PBl 188 and PB 1189, the pLDR8 (ATCC # 77357) as a template and Phusion DNA polymerase (Finnzymes, Finland). The two PCR products (respectively 151 and 729 bp) were purified and used as templates in a second PCR step with the PB 1186 and PB 1189 primers and the Phusion DNA polymerase. Two independent PCR products (880 bp) were pooled and ligated into the pCRII blunt vector to obtain plasmid pCRblunt + PBl 186-1189 (4400 bp). Clones of pCRII blunt + PBl 186- 1189 were selected following an EcoRV/Spel digestion. pCRII blunt + PBl 186-1189 was digested with EcoRV and Spel in order to obtain the EcoRV-Spel fragment A (936 bp). pPB837.2 was digested with EcoRV and Spel and fragment EcoRV-Spel was isolated from agarose gel (3464 pb: fragment B). Fragments A and B were ligated to generate plasmids pPB838 (4216 bp). Clone pPB838 was selected following a HindIII digestion. The integrity of the sequence of cl gene and Pl promoter was confirmed by sequencing. pPB885 plasmid (pMCS5 + Cat without bla promoter) + Pl::cl (FIG 24) This plasmid was deprived from pPB838 plasmid where the chloramphenicol acetyl transferase gene (cat) was removed by Avrll restriction digestion (FIG 24). pPB885 plasmid contains the cl gene under the control of the weak promoter of kanamycin gene (Pl). The pPB885 plasmid was isolated as described in the example 4 (see below) following transformation of the ApurN λPr::sacB Qkan E. coli host strain by direct selection on LB agar plate supplemented with 10% sucrose. plasmids pPB844-pPB845 (FIGS 23-24)

The DNA fragment corresponding to the cl gene under Pl promoter was obtained by PCR using primers PBl 186 and PB1263 (SEQ ID NO:41)

(TCAGCCAAACGTCTCTTCAGGCCAC), the pPB838 plasmid as a template and Phusion DNA polymerase. Four independent PCR products PB 1186-PB 1263 (864 bp) were pooled and ligated into the pCRblunt vector to obtain plasmid pCRblunt + PBl 186-1263 (4383 bp). Clones of pCRblunt + PBl 186-1263 were selected following a HindIII digestion.

The pCRblunt + PBl 186-1263 plasmid was digested with EcoRI and the extremities were filled with Klenow Polymerase to generate a blunt fragment (929 bp : fragment A). The pPB828 plasmid was digested with Xmnl and the resulting Xmnl-Xmnl fragment was purified (fragment B: 4135 bp). Fragments A and B were ligated to generate plasmids pPB844 or pPB845. Candidates were screened by a BamHI/Hindlll digestion. In the pPB844 plasmid (see FIG. 20), the cl gene is in the same direction as the transgene of interest to be cloned into the MCS (multiple cloning site) of the vector under the control of the CMV promoter (SEQ ID NO:43). In the pPB845 plasmid (see FIG. 21) the cl is oriented in the opposite direction.

C. Obtaining the cl gene with its native promoter plasmids pPB846-pPB847 The DNA fragment corresponding to the cl gene under its own promoter (cl native promoter: SEQ ID NO:30) was obtained by PCR using primers PB 1266 (SEQ ID NO:42) (GCTGACTCATACCAGGCACGCACGGTGTTAGATATTTATCCC) and PB 1263, using pLDR8 plasmid as a template and the Phusion DNA polymerase. pPB846-pPB847 plasmids Four independent PCR products PB1266-PB1263 (777 bp) were pooled and ligated into the pCRblunt vector to obtain plasmid pCRblunt + PB1266-1263 (4296 bp). Clones of pCRblunt + PB1266-1263 were selected following a HindIII digestion.

The pCRblunt + PB 1266- 1263 plasmid was digested with EcoRI and the extremities were filled with Klenow Polymerase to generate a blunt fragment (842 bp : fragment A). pPB828 plasmid was digested with Xmnl and fragment Xmnl-Xmnl was purified (fragment B: 4135 bp). Fragments A and B were ligated to generate plasmids pPB846 (4999 bp) or pPB847. Candidates were screened by a BamHII HindIII digestion. In the pPB846 plasmid (see FIG. 22), the cl gene is in the same direction as the gene of interest to be cloned into the MCS site of the vector under the control of the CMV promoter. In the pPB847 plasmid (see FIG. 23), the cl gene is oriented in the opposite direction.

D. CONSTRUCT RESULTS

FIG. 7 shows the constructs of pPB838 and pPB829 harboring the cl repressor that were used to prepare the drugless plasmid. Both plasmids were marked with the Chloramphenicol (Cat ^R ) gene. The Cat gene on these plasmids were required only for determining proof of concept but can be finally removed (see FIGs 13-15 and Example 4).

Example 4: Transformation of the E. coli host strain with the cl plasmid(s) and direct selection on a sucrose agar plate An experimental procedure that enables the screening of the transformed cells by the chloramphenicol (Cat) marked cl plasmid(s) without the use of antibiotic as selection pressure on agar plate(s) has been efficiently demonstrated (see FIG 13-15). Competent cells of E. coli λPr::sacB ⁺ ΔpurNΩ.Kan were transformed with 1 μg of either pPB838 or pPB829 plasmid and incubated for 3 hours at 3O ⁰ C in LB liquid broth medium. 100 μl aliquot of diluted (10 ^~3 and

10 ^"4 ) transformed cells (with pPB838 and pPB829 plasmids) were spread onto LB agar plate containing 10% sucrose. These dilutions ranging from 10 ^"3 to 10 ^"4 appear appropriate to select true transformant cells (e.g. having cl plasmid) since 98-100 % of sucrose resistant cells growing on LB agar plate containing sucrose were Cat ^R . This suggests that these transformed cells have the cl plasmid (pPB829 or pPB838).

FIG 13A shows that competent cells of E. coli λPr::sacB ⁺ ΔpurNΩ.Kan were transformed with 1 μg of either pPB838 or pPB829 plasmid and incubated for 3 hours at 3O ⁰ C in LB liquid broth medium before dot-spotting (4 μl for each dot). The figure shows the Log dilutions (ranging from undiluted to 6 Log dilution) on LB agar plates containing either chloramphenicol (15 μg/ml) or sucrose (10%). The untransformed competent parental strain cells of E. coli λPr::sacB ⁺ were used as standard cells grown under similar condition. As expected, the standard cells did not grow on selective plates such as cat and sucrose. Nevertheless, due to the long period of incubation (3 days), some spontaneous mutant resistant to sucrose appeared under this condition. This level of spontaneous mutant defines a baseline to be considered for the screening of true transformant cells. Two Log differences were observed with transformed cells growing on cat and sucrose under this experimental condition. Competent cell, per definition, presents a temporally of cell wall weakness. The post-electroporation incubation of transformed cells in LB liquid medium enables the regeneration of both the aforementioned cell wall weakness and the induced-pores of the cell wall and cell membrane following electroporation. Since the competent cells were genetically sensitive to sucrose due to the λPr::sacB ⁺ ΔpurNΩ.Kan cassette insertion, the transformed cells recovery in the presence of sucrose was reduced by two Logs compared to the cells recovery on Cat. These observations were confirmed by spreading 100 ul of transformed cells on selective plates (see FIG 13B).

FIG 13B shows that 100 μl of transformed (with pPB838 and pPB829 plasmids) and untransformed standard cells were spread on selection LB agar plate containing either cat or sucrose using appropriate Log dilution to determine the rate of transformation efficacy on cat and sucrose and standard spread on sucrose to better define the baseline of spontaneous mutant rate to be considered for successful screening of true transformant cells capable of growth on LB sucrose agar plate. In agreement with the previous dot-spotting cells experiment (see FIG 13A), spontaneous mutant resistant to sucrose were not detectable when the 10 ^"3 dilution of transformant cells were spread on LB agar plate containing sucrose. As previously shown (see FIG 13A), there was 2 Logs difference in the transformation efficacy between selection on cat and sucrose under this condition. The dilutions ranging from 10 ^"3 to 10 ^~4 when spread on LB agar containing sucrose appeared appropriate to select true transformant cells (e.g. having cl plasmid).

FIGS 14A-C illustrate the efficacy of the experimental procedure to select transformed cells with the cl plasmid only (pPB829 and pPB838) without the use of antibiotic as selection pressure. FIG 14A shows the experimental overview using replica plate assay to validate the screening of true transformant cells having the cl plasmid. Similar to what is shown in FIG 13B, parental competent cells λPr::sacB ⁺ ApurNΩ.Kan were transformed with the cl plasmid (pPB829 and pPB838) and appropriate Log dilutions were spread onto selective (Cat or Sucrose) LB agar plates before incubation for 3 days at 3O ⁰ C. Afterwards, the plates showing CFUs (colony forming units) titers ranging from 20 to 150 were replicated on new fresh LB agar plates containing either Cat or sucrose or a combination of both before incubation for 3 more days at 3O ⁰ C. In theory, Cat resistant colonies should correspond to cells propagating the cl plasmid(s).

FIG 14B shows that experimental results confirmed that the Cat ^R CFUs were identical between the number obtained on sucrose LB agar plates and on Cat/sucrose LB agar plates after replica plate assay. As demonstrated in FIG 14B (panel B), the CatR CFUs are i) all sucrose resistant and ii) all resistant to the combination of Cat and Sucrose, signifying that all parental cells following transformation harbor the cl plasmids. Panel C shows that the 10 ^"3 and 10 ^"4 dilutions of transformant cells selected directly on sucrose were also resistant to sucrose and to the combination of both sucrose and Cat, also signifying that these sucrose resistant CFUs correspond to true transformed cells containing the cl plasmid. Indeed, when non transformed parental cells were spread onto LB agar plate containing sucrose, the sucrose resistant CFUs when replicated on a combination of sucrose and Cat did not grow as expected because the chloramphenicol resistance is a phenotypical trait brought by either the pPB829 or pPB838 plasmid.

FIG 14C represents the statistical data obtained from the replica plate assay (see FIG 14B) and more particularly the percentage of the colonies having the cl plasmid that were selected without the use of antibiotic. Under this experimental condition, the percentage of colonies having the cl plasmid ranges from 98% to 100%.

Example 5: Different culture conditions with and without sucrose showing that the cl plasmid can be maintained E. coli cells expressing sacB, ΔpurN λPr::sacB ⁺ Ω.kan were grown in the presence or absence of sucrose in order to check the functionality of the λPr::sacB ⁺ cassette inserted into the chromosome of E. coli.

FIG. 4 describes the "sucrose sensitivity" of the inactivated system wherein a switch to 42°C in temperature inhibits the binding activity of cl repressor to the λ Pr promoter and renders the host cells sensitive to sucrose. The cl repressor protein, expressed from the plasmid, inhibits the transcription of the toxic sacB gene product placed under the λ Pr promoter that is located on the host cell chromosome. The host E. coli cells' viability in the presence of sucrose is ensured by a sufficient expression level from the plasmid of the λ cl repressor protein. FIG. 5 describes the combination of the host strain with the antibiotic-free pDLl plasmid in the presence of sucrose to yield drugless plasmid (pDL: DrugLess Plasmid). The experimental plan for the proof of concept is further illustrated in FIGs 6 A, 6B, and 6C.

The physiological characteristics of host cells expressing sacB, ApurN Ω λPr::sacB ⁺ Ω.kan when grown in the presence or absence of sucrose are described in Table 1. Table 1. The physiological characteristics of sacB ⁺ host cells, ApurN Ω λPr::sacB ⁺ , when grown with sucrose and without sucrose

The physiological characteristics of host cells expressing sacB (λPr::sacB ⁺ ), with and without pDLl plasmid when grown in the presence or absence of sucrose, are described in Table 2.

Table 2. The physiological characteristics of (λPr::sacB ⁺ host cells with and without pDLl plasmid when growing with sucrose and without sucrose.

FIGs 10-11 provide results confirming that the genetically engineered host strains were highly resistant to sucrose at 30-37 ⁰ C, while no growth was observed at 42°C. FIG. 10 shows that second passage with the E. coli host strains, ΔpurN λPr: :sacB ⁺ Ωkan, harboring either the pPB829 plasmid or pPB838 plasmid grew well in the presence of sucrose at 32 ⁰ C. The sacB gene expression was perfectly repressed by the cl gene product synthesized from both c/plasmids. The plasmid maintenance was 100% efficient (versus control on Cat ^R ) and cells were able to survive in the presence of sucrose. At 42 ⁰ C, the cl gene product was inactivated. The plasmid could not be maintained and the cells died in the presence of sucrose. This demonstrates that the cells viability in the presence of sucrose was due to the presence of a functional cl plasmid.

Example 6: Culture conditions showing that the cl plasmids are stable over 5 passages, without antibiotic selection pressure, and lack of spontaneous mutants for sucrose resistance Once the drugless plasmids were produced, they were tested for stability and their robustness over several passages in fresh broth medium containing either sucrose or Cat.

FIG. 11 illustrates the stability of the pPB829 and pPB838 plasmids in growing cells, including lack of obvious genetic rearrangement during selection on either chlorampenicol or sucrose. Comparable plasmid yields and maintenance in cultures growing in the presence of sucrose and chloramphenicol were observed even after five successive passages. The antibiotic and non antibiotic systems were at least equivalent in terms of plasmid maintenance and selection from growing cells.

FIG. 12 illustrates the lack of spontaneous mutations in the λPrr.sacB cassette inserted in the parental E. coli strain when cells were growing at 3O ⁰ C. This observation clearly demonstrates the cl plasmid stability in such E. coli genetic background when cells were growing in presence of sucrose (see FIG 10). This mutation rate experiment was performed with the parental strain (purN deleted expressing the sacB gene). It was shown that no mutation occurred at 3O ⁰ C after 18h incubation. Clonal cultures of parental cells harboring the sacB ⁺ cassette in their chromosome were grown in LB containing sucrose. Cells were harvested at an OD of 1.0 and aliquots of 100 μL were spread onto LB agar plates containing sucrose. Since the presence of the sacB gene causes the cells death, the spontaneous mutation rate was evaluated by scoring the sucrose resistant colonies after overnight incubation at 3O ⁰ C. Standard cells harboring the sacB ⁺ cassette but transformed with the c/plasmid (pPB829 or pPB838) became resistant to sucrose and culture of DO 1.0 needed to be diluted by 5 Log before being spread (100 uL) onto LB agar plates supplemented with sucrose for scoring. Following the overnight culture (18 hours incubation), no sucrose resistant colony was detected with the parental cell cultures (ApurNλPr: :sacBΩ.kan) while the 5 Log diluted cultures of ApurNλPr: :sacBΩ.kan transformed with the pPB829 or pPB838 plasmid(s) grew. Thirty-six hours were required to detect some sucrose resistant colonies (ApurNλPr: :sacBΩ.kan) when the cells were incubated at 37 ⁰ C.

Spontaneous mutation occurred at a rate of 1 mutation/1.3 X 10 ⁵ cells after 36 hours when incubated at 37 ⁰ C. The cl plasmid maintenance in the cells growing in the presence of sucrose was effective after overnight incubation at 30°C while no growth of the mother strain was detected.

Example 7: Improvement of the Parental E. coli host strain to reduce the rate of the spontneaous mutation to sucrose by adding a second sacB cassette (AedAλPr::sacBΩ.cat) into the aforementioned one sacB cassette E. coli host strain (ApurN λPrr.sacBQkaή).

Both FIG 8C and FIG 9D illustrate how the new parental host strain of E. coli harboring two sacB cassettes (ApurN λPrr.sacBQkan AedA λPr::sacBΩ.cat) were engineered.

FIG lOB-C demonstrate that the dual sacB cassettes E. coli strain (ApurN λPr: : sacB ⁺ Ωkan AedA λPr::sacB ⁺ ΩCat) was highly sensitive (no growth) in the presence of sucrose when incubated at 3O ⁰ C and 42 ⁰ C while some spontaneous mutants appears with the one sacB cassette E. coli strain plated on even 10% sucrose (ApurN λPr: :sacBΩ.kan). FIG 1OC also illustrates that the lowest sucrose concentration required(2% final) for the dual sacB cassettes E. coli strain compared to the one sacB cassette E. coli strain (10% final). Moreover, it appears that the two sacB cassettes E. coli strain was much more sensitive to sucrose, as shown at low sucrose % and at the highest temperature such as 42 ⁰ C. This suggests that temperature at about 37 ⁰ C and sucrose concentration at about 2% (or slightly below) may correspond to the optimal condition for maintaining the plasmid(s) harboring the cl gene repressor in growing cells. FIG 12B-C demonstrate that the presence of two sacB cassettes (rather than one) confers a better robustness to sucrose sensitivity (still optimal at 2% sucrose) at temperatures ranging from 3O ⁰ C to 37 ⁰ C. The E.coli cells transformed with plasmids containing two sacB cassettes BJ5183 ApurNΩ λPrr.sacB Km AedaΩ λPrr.sacB Cat clone #1 and BJ5183 ApurNΩ λPrr.sacB Km AedaΩ λPrr.sacB Cat clone #2, and E.coli cells transformed with plasmids containing one sacB cassette BJ5183 ApurNΩ λPrr.sacB Km clone #16 and BJ5183 AedAΩ λPrrsacB Km clone #11 were grown to OD600 about 1.0. About 100 ul of each culture was spread onto LB/agar plates containing 0%, 2% and 4% sucrose (dilution 10 ^"5 ) for CFU scoring. The plates were incubated at 3O ⁰ C to 37 ⁰ C. Incubation at 37 ⁰ C was used to evaluate the robustness of the parental strain expressing the dual sacB cassettes in the presence of 2% and 4% sucrose. FIG 12B-C and Tables 3-7 below show that the presence of one sacB cassette in the E. coli chromosome was less robust at sucrose concentration ranging from 2 to 4% at similar temperatures such as 3O ⁰ C and 37 ⁰ C. As confirmed in this plating assay, the mutation rate to sucrose with the two sacB cassettes E. coli strain was undetectable at the lowest sucrose concentration (2%) when incubated at 37 ⁰ C while the mutation rate with the one sacB cassette E. coli strain was ranging from 3.8 10 ^"6 to 5 10 ^"5 . An independent set of experiment showed that the mutation rate in the two sacB cassettes E. coli strain was nearly 5.10 ^"10 when incubated at 37 ⁰ C in presence of 2% sucrose.

Table 3

Table 4

Table 5

Table 6 Table 7

1*: BJ5183 ApurNΩ λPrrsacB Km AedaΩ λPrr.sacB Cat clone #1 2*: BJ5183 ApurNΩ λPrr.sacB Km AedaΩ λPrr.sacB Cat clone #2 3*: BJ5183 ApurNΩ λPrr.sacB Km clone #16 4*: BJ5183 AedAΩ λPrr.sacB Km Clone #11

Example 8: Plasmid maintenance and yields obtained in the absence of antibiotic selection pressure (the cl plasmid is maintained at a high copy number in the host strain)

E. coli host cells expressing sacB and harboring the cl plasmid (pPB829 or pPB838) were grown in the absence of antibiotic selection pressure (e. g. in presence of sucrose) or in the presence of Cat as control.

In FIG. 11, the plasmid yield was determined at passages 2 and 5. Plasmids were extracted from cultures maintained at 3O ⁰ C and subjected to gel electrophoresis. The HindII restriction digestion indicates that no obvious genetic rearrangement occurred in either pPB829 or pPB838 during propagation and selection with either Cat or Sucrose. Moreover, similar cl plasmids maintenance was observed during the selection of the cl plasmids under either Cat or Sucrose selection pressure.

FIGs. 15A-B illustrate the stability and efficacy of the cl plasmids (pPB829 and pPB838, containing one copy of the sacB cassette) maintenance in cultures, including lack of genetic rearrangement after selection on either chlorampenicol or sucrose. Efficient plasmid maintenance and the stability of cl plasmids grown in the presence of either sucrose or chloramphenicol were observed. Moreover the plasmid yield (copy number) was much higher (approximately 700 per cell) when parental host strain transformed with either the pPB838 or pPB829 were grown at 3O ⁰ C in the presence of sucrose as selection pressure than cultivated in the presence of Cat as the selection pressure. The plasmid maintenance and yield were highly efficient and the copy number was twice higher compared to cells cultivated in the presence of Cat. This better cl plasmid yield when host cells were cultivated in the presence of sucrose was due to the weak promoter (Pl) that controls the cl expression. Indeed, the expression of the cl repressor under the control of the weak promoter of Kanamycin gene (Pl) had a positive effect on the plasmid yield to better counteract the toxicity of the sacB gene placed under the control of the λPr promoter.

As earlier demonstrated (see FIG HA), the antibiotic and non antibiotic systems are equivalent in terms of plasmid maintenance and selection of growing cells harboring the cl plasmid. FIG 15B demonstrates that the plasmid yield (copy number) was much higher

(approximately 700 per cell) when parental host strain transformed with either the pPB838 or pPB829 were grown in the presence of sucrose as selection pressure rather than cultivated in the presence of Cat as the selection pressure. The plasmid maintenance and yield were highly efficient and copy number was twice higher compared to cells cultivated in the presence of an antibiotic selection, such as Cat.

Example 9: Plasmid maintenance and yields in the absence of antibiotic gene in the cl plasmid: comparative analysis between both one and two sacB cassettes E. coli strains Figures 15C-E illustrate the stability and yield of cat-free pPB885 plasmid in both one and two sacB cassettes E. coli strains {ApurN λPr::sacB ⁺ Ωkan and ApurN λPr: :sacB ⁺ Ωkan AedA λPr::sacB ⁺ Ωcat, respectively). Both ApurNΩλPr::sacB km (one sacB cassette) and ApurNΩλPr: :sacB km AedaΩλPr: :sacB cat (two sacB cassettes) parental strains were transformed with the cl cat-free pPB885 plasmids and incubated for 3 hours in LB prior to plating on sucrose plate and subsequent cultivation (3 passages) in LB liquid growth medium supplemented with sucrose. The plasmid yield was determined at passages 3 in LB supplemented with 5% and 10% sucrose for the two and one sacB cassettes E. coli strains, respectively. The pPB885 plasmids were extracted from cultures maintained at 37 ⁰ C and subjected to gel electrophoresis. The PvuII restriction digestion indicates that no obvious genetic rearrangement occurred in either pPB885 during propagation and selection with Sucrose (see FIG 15D). As demonstrated in this gel, no obvious genetic rearrangement occurred after selection with sucrose at either 10% and 5% in both E. coli host strains, one and two sacB cassette, respectively.

Figure 15E illustrates the yield of the cat-free pPB885 plasmid in both one and two sacB cassettes E. coli strains (ApurN λPr::sacB ⁺ Ωkan and ApurN λPr::sacB ⁺ Ωkan AedA λPr: : sacB ⁺ Ωcat, respectively). Interestingly, the plasmid yield (copy number) was much higher (approximately 278 per cell) in the two sacB cassettes compared to the plasmid yield (approximately 142 per cell) obtained from the one sacB cassette E. coli strain. The plasmid yield obtained in this experiment appears lower than the plasmid yield obtained in the previous comparative study (see FIG 15B). This is mainly due to the fact that i) more clones need to be evaluated following transformation with the pPB885 to identify the one being the best and ii) moreover, the growth condition of the two sacB cassettes E. coli strain transformed with the cat-free pPB885 plasmid was not optimally cultivated. Indeed, we had demonstrated that the optimal growth condition for this latest strain was at about 37 ⁰ C and about 2% sucrose (or slightly below). However, this result suggests that under optimal robustness condition, the plasmid yield could be better in a two sacB cassettes E. coli strain compared to the E. coli strain having one sacB cassette.

Example 10: Antibiotic-free cl plasmid transfection in CHO cells

Figure 15F and Table 8 below illustrates the plasmid transfection efficacy (GFP expression) in CHO cells of the antibiotic-free plasmid (pPB896 /Sucrose as selection pressure) and antibiotic plasmid (pCG105 / Cat as selection pressure). No obvious difference is observed in terms of expressed GFP protein between these two selection pressures (Sucrose versus Cat as antibiotic). The pCG105 plasmid (pPB828-derived plasmid having the GFP gene placed under the control of the CMV promoter [Merial property]) and the pPB896 (pPB829 plasmid + Pl::cl gene + GFP) plasmids were used in CHO cells' transfection to evaluate the functionality of the antibiotic-free plasmid concept versus current antibiotic plasmid. This experiment was performed using the plasmids isolated from culture of E. coli host strain having one sacB cassette. The pCG105 was propagated in E. coli using Kanamycin as selection pressure while the pPB896 was propagated in E. coli using 10% sucrose as selection pressure. DNA plasmids were extracted and dosed at the same concentration. FIG 15F illustrates the in vitro expression of the GFP protein encoded by pCG105 and pPB896 after transient transfection of CHO-Kl cells, using Lipofectamine 2000. CHO-Kl cells at 90% confluency in 6 cm diameter plates were transfected with 5 μg plasmid and 10 uL Lipofectamine each, according to manufacturer's instruction. After the transfection, cells were cultivated in MEM-glutamax medium containing 1% SVF for 24 hours. Culture were harvested and analyzed using fluorescence microscopy (FIG 15G) and Flow Cytometry (FACS Calibur [Becton Dickinson]) (Table 8).

FlG 15F shows that similar GFP epifluorescence was visualized in both transfected CHO-Kl cells. The pCG105 and pPB96 plasmids under the selection pressure of chloramphenicol and sucrose were able to transfect the CHO-Kl cells with comparable efficiency.

Table 8 shows the efficacy of CHO transfection with the pCG105 and pPB896 plasmids. As indicated in this table, a comparable % of CHO cells expressing the GFP protein was determined when CHO was transfected with pCG105 (59% GFP expressed) and pPB896 (48% GFP expressed). These results validated the use of the sucrose-based antibiotic-free plasmid system for application in CHO transfection and DNA vaccine purposes, ultimately.

Table 8

These Examples demonstrate that the control of a toxic gene located on the chromosome can be achieved with a repressor located on a plasmid. The system is fully functional in the absence of antibiotic selection pressure. The system is stable over five (5) passages of the host cells. The system allows host cells to achieve a high plasmid copy number per cell. The system is fully compatible with the use of a minimum synthetic culture medium.

* * * Having thus described in detail preferred embodiments of the present invention, it is to be understood that the invention defined by the above paragraphs is not to be limited to particular details set forth in the above description as many apparent variations thereof are possible without departing from the spirit or scope of the present invention.

All documents cited or referenced herein ("herein cited documents"), and all documents cited or referenced in herein cited documents, together with any manufacturer's instructions, descriptions, product specifications, and product sheets for any products mentioned herein or in any document incorporated by reference herein, are hereby incorporated herein by reference, and may be employed in the practice of the invention.