The present invention relates to an assay method for detection of specific nucleotide sequences in DNA or RNA, and to kits useful for performing such assays. Background Of The Invention

Molecular hybridization is extremely useful in the detection of specific nucleotide sequences in genetic materials. Generally, single-stranded chromosomal DNA from a test specimen is denatured and attached to a DNA- binding membrane or filter support. The membrane-filter is brought into contact with a labeled single-stranded polynucleotide probe under conditions promoting hybridizatio of complementary DNA sequences. Double stranded "duplex" or "hybrid" molecules are detected by a variety of technique depending on the nature of the particular label used. In order for hybridization to occur, the probe-molecules must contain sequences substantially complementary to those sequences in the nucleic acid to be detected. An example of such a hybridization system is U.S. Patent No. 4,358,535 to Fal ow et al.

Until recently, hybrid molecules have been detected exclusively with radioisotopically labeled probes. The presence of hybrids is detected by scintillation counting or autoradiography, thereby providing a quantitative assay for the presence of the specific DNA of interest. Although autoradiography is a senstitive method, it can be time consuming. Moreover, hybrid-detecting methods involving

the use of radioisotopes must be performed under stringent safety precautions. Radioisotopes are extremely expen¬ sive, and in some cases have a limited shelf life due to rapid disintegration.

A currently used method for radioactivly labeling DNA probe molecules incorporates 32p__ 14r_. _or 3H_ _con _ taining nucleotides by the "nick translation" method of Rigby et al., J. Mol. Biol. 113:237 (1977). According to this technique, nuclease treatment of the probe nucleic

10 acid is necessary to nick or open gaps in one of the strands. A radiolabelled nucleotide is then inserted into the DNA with a polymerase.

A more recent innovation in molecular genetic probe technology involves utilization of the nick-translation

15 technique to incorporate biotinylated dUTP into probe DNA.

The nick-translation technique requires nuclease ^• treatment of the template nucleic acid to open gaps in one of the strands. A radiolabelled or biotin-substituted nucleotide is inserted with a polymerase.. The duplex is

20 then split with the thus-labelled strand being used as a probe.

Biotin-labeled DNA may be detected by a soluble complex of biotin-bound horseradish-peroxidase and strepta- vidin, or by means of a biotin-specific antibody, followed

25 by a secondary fluorescein-labeled antibody.

A more recent technique dispensing with the need for nick-translation is described in European Patent Application Publication 128018. Nucleotides of the probe molecule are modified iι situ, for example, by alkylation. Antibodies

^• 30 to the alkylated probe are then used to detect the formation of duplexes.

Yet another chemical-modification approach is disclosed in European Patent Application Publication 122614. A chem¬ ically-labeled nucleotide, e.g., a biotinylated nucleotide,

35 capable of acting as a substrate for terminal deoxynucleotide transferase, is polymerized onto the terminal end of the

probe molecule. A polymer with a biotin-containing analog of TTP is formed on the 3'-OH terminus of the probe molecule. Antibodies to the probe molecule may be used to detect the presence of hybrid molecules. The above methods, relying on the insertion of radio- labeled or chemically-modified nucleotides, on chemical modification of probe DNA molecules, or on terminal poly¬ merization of biotinylated nucleotides, are costly and time consuming. Although non-isotopic labelling of nucleic acids has been successfully employed in molecular hybridi¬ zation and diagnostic tests, Langer et al., Proc. Natl. Acad. Sci. USA, 6_8:6633 (1981), Singer et al., Proc. Natl. Acad. Sci. USA, 7_9:7331-35 (1982), Hutchinson et al., J. Cell. Biol., 9J5:609-618 (1982), the key sub- stance, biotinylated dUTP, is a high-cost product because of its intricate chemical synthesis.

Antibodies to UV-irradiated DNA have been prepared by Mitchell et al., Biόchem. Biophys. Acta, 655;54-60 (1981) and} Eggset et al., Carcinogenesis _4:745-750 (1983). However, such antibodies have not heretofore been used in conjunction with molecular probes to detect the presence of specific nucleotide sequences.

What is needed is a simple but sensitive method for detecting DNA which dispenses with the need for time con¬ suming and costly radiolabelling or intricate chemical synthesis of specialized nucleotides.

Summary Of The Invention

A method for detecting the presence of specific nucleic acid in a sample is provided. A polynucleotide probe which will hybridize with the suspect nucleic acid is prepared. The probe is labelled by inducing the formation of UV-photo- products by exposure to ultraviolet radiation. The photo- products are recognizable by antibodies to the UV-labelled probe. The labelled probe is then contacted with substantial single-stranded nucleic acid from the sample under hybridizin conditions. Hybrid complexes are detected with anti-UV-nucle

acid antibodies labelled with a signalling means. The anti¬ bodies bind the UV-labelled probe to indicate the presence of the suspect nucleic acid in the sample.

In one embodiment of the invention, the labelled 5 probe includes a polypyrimidine nucleotide sequence or "tail" at the 3'-OH terminus. UV-photoproducts, principally pyrimidine dimers, are created in the tail by exposure to UV-radiation. The tail is generated by cloning the probe directly into a polypyrimidine nucleotide sequence or by ° polymerizing pyrimidine nucleotides onto the 3*-OH terminal end of the probe with the enzyme terminal transferase.

The present invention also relates to a method for detecting total nucleic acid. A sample purportedly con¬ taining nucleic acid is irradiated with ultraviolet light ° to form UV photoproducts in nucleic acid contained within the sample. The irradiated sample is contacted with anti-UV- nucleic acid antibodies. A signalling means associated with the antibodies signals their binding to - nucleic acid in the sample. 0 The invention also relates to a kit for detecting the presence of specific nucleic acid in a suspect sample. The kit contains a supply of UV-labelled nucleic probe molecules containing UV-photoproducts. The probe is selected so as to hybridize with the nucleic acid to be detected. The 5 kit further contains means for contacting the suspect sample with the UV-labelled probe to form hybrid complexes. The- kit contains a supply of anti-UV-nucleic acid antibodies which bind the UV-labelled probe. The anti- UV-nucleic acid antibodies are labelled with a signalling 0 means. The kit may contain a means for measuring the signal to indicate the presence of or extent of the nucleic acid being detected.

Yet another kit, which may be used to quantify total DNA in a test sample, includes a supply of anti-UV-DNA 5 antibody, a panel of DNA-containing samples of known DNA concentrations as standards, a signalling means for indicatin binding of anti-UV-DNA antibody, and means for comparing

the level of the signal generated by DNA in the test sample with the levels of signal generated by the various members of the standard panel.

It is therefore an object of the invention to pro- vide a method for detecting or quantifying the presence of specific nucleic acids in a sample.

It is an object of the invention .to provide a method for detecting specific nucleic acids without the need for radioactive labelling or chemical synthesis of biotinylated or other synthetic nucleotides.

It is an object of the invention to pro ide a method for detecting nucleic acids which may be used to detect the presence of organisms containing either single-stranded or double-stranded genetic material. it is an object of the invention to provide a method for quantifying total DNA in a sample.

It is a further object of the invention to provide a test kit for the detection or quantification of specific nucleic acids. * ^•

It is an object of the invention to pro ide a kit for detecting the amount o£ total DNA in a test sample.

These and other objects are apparent from the following disclosure.

Brief Description of the Figures

Figure 1 contains chemical structures of UV-induced DNA-photoproducts which may be generated in probe molecules according to the present invention.

Figure 2 is a schematic sequence illustrating the process of -the present invention for photoimmune detection of DNA wherein a single-stranded DNA-probe has been extended with a poly-dT tail sequence before UV-irradiation to increase the number of UV-photoproducts for antibody binding.

Figure 3 is a photograph of the hybridization of a representative UV-irradiated DNA probe to homologous DNA which is attached to a membrane filter. Hybrids were detected with rabbit anti-UV-DNA antibody and biotinylated

goat-anti-rabbit IgG, followed by incubation with strep- tavidin and biotinylated alkaline phosphatase.

Figure 4 is a photograph of the hybridization of a UV- irradiated bacteriophage Hyl7 DNA-probe to different con- centrations of single-stranded H l7 DNA bound to a filter membrane in the presence of the negative controls bacterio¬ phage lambda-DNA and calf-thymus-DNA.

Figure 5A is a photograph of a conventional ethidium bromide staining of restriction enzyme-digested DNA followin agarose gel electrophoresis.

Figure 5B is a photograph of the same DNA fragments following alkaline denaturation, transfer to a filter- membrane by electroblotting, and incubation with anti-UV- DNA-antibody conjugated to horseradish peroxidase. Detailed Description of the Invention

By "UV-photoproduct" as used herein is meant a nucleo¬ tide sequence which has undergone chemical modification in one or more nucleotides by exposure to ultraviolet radiation

By "UV-nucleic acid" or "UV-DNA" is meant nucleic acid _or DNA which has been exposed to ultraviolet radiation thereby causing the formation of UV-photoproducts.

By "anti-UV-nucleic acid antibody" or "anti-UV-DNA antibody" is meant antibodies which bind UV-nucleic acid or UV-DNA but not normal nucleic acid or normal DNA.

The present invention is based upon a form of _in situ- labelling of DNA by means of UV-light which occurs in a one—step process, as distinguished from the incorporation of radiolabelled or chemically-labelled nucleotides using the nick-translation method, or the terminal polymerization of such labelled nucleotides to probe molecules.

A polynucleotide probe molecule is prepared which con¬ tains a nucleotide sequence substantially complementary to t nucleotide sequence of the unknown genetic material to be detected. The probe comprises a single strand, or partially denatured double strand, of nucleic acid. The probe may be DNA or RNA, which is extracted from the relevant genetic material and cloned or synthesized chemically when the nucle tide sequence is known. According to U.S. 4,358,535, probe

molecules may be obtained from messenger RNA, from cDNA generated by reverse transcription of messenger RNA, or from cleavage of the genome, followed by cloning of the probe molecule in accordance with existing techniques, e.g. Mani- atis, T., et al.. Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Laboratory (1982), p. 187, 211.

The DNA-probe molecules are irradiated with a relativel low dose of ultraviolet radiation of between 200-400nm wavelength, for example, 2500 J/m^ at 254nm, in order to form UV-photoproducts. Most of the photoproducts thus formed are pyrimidine dimers, but a few saturated pyrimidine products also occur. See Figure 1. Binding occurs between the C-5 position in a first pyrimidine and the C-5 position in a neighboring pyrimidine base, or between the C-6 position in the first pyrimidine base and the C-6 position in the neighboring pyrimidine base. Binding may also occur between the C-6 position of the pyrimidine. base closest to the 3'-OH terminal end of the probe and the C-4 positio ^' of the i neighboring pyrimidine base.. Irradiation can also include saturation of the double bond between the C-5 and C-6 position of any pyrimidine ring through UV-induced uptake of hydrogen and/or hydroxyl. The DNA probe molecules bearing the aforementioned photoproducts are potent antigens to which antibodies may be raised. Photoproducts, can, as an alternative, be produced by UV-light in combination with a photoreactive chemical compound such as furocoumarin.

The sample material to be assayed comprises nucleic acid, either single or double stranded. When assaying for nucleic acid form organisms containing double-stranded nucleic acids, a denaturing step is required. Denaturing techniques are well-known in the art and do not form part of the present invention. See, for example, Maniatis, T., et al. , Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Laboratory (1982), p. 314, 321, 383; Gergen et al.. Nucleic Acids Res. 7 _ 2115-2136 (1980). Heat denaturation at 95-100°C for five minutes, followed by rapid chilling is the preferred method of denaturation.

The irradiated probe-DNA is incubated with the sample nucleic acid material containing the purported nucleotide sequence. According to one technique, the sample nucleic acid is affixed to a support material capable of binding nucleic acid. Cellulosic supports are preferred, partic¬ ularly nitrocellulosic filter papers, which readily bind DNA. One such material is the "Gene Screen" hybridization transfer membrane available from New England Nuclear. It should be understood, however _/ that attachment to a solid support material is merely one way of visualizing hybridi¬ zation, it being contemplated that the hybridization process may also be performed in solution.

Hybridization per se is a well-known technique and is described in the literature. See, for example. Molecular Cloning, supra; U.S. 4,358,535. The particular hybridiza¬ tion technique utilized will depend upon the nature of the sample material.

UV-photoproducts in DNA do not decrease hybridization capacity significantly when the probes are greater than 100 bases. Smaller probes will also hybridize well with a low UV-dose.

The formation of hybrid complexes between the probe- nucleic acid and nucleic acid in the test specimen is detected by anti-UV-nucleic acid antibodies which are labelled with a suitable signal means. Anti-UV-nucleic acid antibodies may be produced according to Mitchell, D. L. and Clarkson, J. M. , Bioche . Biophys. Acta, 655: 54-60 (1981) or Eggset et al, Carcinogenesis 4_:745-750 (1983), which disclose immunization of rabbits with single-stranded UV-irradiated calf thymus DNA conjugated to methylated bovine serum albumin. Antibodies with specificity against un-irradiated DNA may be removed by affinity column chrc-tatograhy through cyanogen bromide activated Sepharose ® (Pharmacia, Sweden ⁾ coupled to un-irradiated DNA (Eggset et al, Supra) . We have found'that the unbound antibody fraction specifi¬ cally binds UV-irradiated DNA with ten times greater affinity against single-stranded DNA compared to double- stranded DNA.

Polyclonal antibodies against UV-nucleic acid pre¬ pared in this manner, or specific monoclonal antibodies thereto, may be used to detect molecular hybridization according to the present invention.

The antibody is labelled with a signalling means. The signal is detected or measured as an indication of the presence or extent of the suspect nucleic acid in the sample. Suitable signalling means used to label the anti- UV-nucleic acid antibodies include chemical labels which are detected on the basis of their own physical properties. Such chemical labels comprise, by way of illustration, and not by way of limitation, color-indicating compounds, particularly fluorescent dyes such as fluoroscein and rhodamine, or reagents of high electron density such as ferritin, hemocyanin and colloidal gold. Alternatively, the signalling means may comprise a substance which is • detected by its binding or reactive properties, such as an enzyme which catalyzes a reaction leading to the generation of detectable reaction products. Examples of such enzymes comprise peroxidase and alkaline phosphatase.

Most advantageously, a secondary antibody which binds to the anti-UV-nucleic acid antibodies is labelled with the signalling means. For example, where the anti-UV- antibodies are raised from rabbits, the secondary antibody may comprise swine-anti-rabbit IgG, conjugated to horse- radish-peroxidase. By the addition of suitable substrates, in this case hydrogen peroxide and diaminobenzidine (DAB), the enzyme will give a color reaction.

The secondary antibody may comprise a biotinylated antibody which binds the anti-UV-nucleic acid antibody and further binds streptavidin, which may in turn be coupled to a signalling means.

The anti-nucleic acid antibodies may be added to the system following a suitable incubation period to allow the probe molecules to hybridize. Alternatively, the anti¬ bodies may be permitted to bind to the probe prior to hybridization.

In one embodiment of the invention, a polynucleotide probe is prepared which includes a polypyrimidine nucleo¬ tide sequence at the 3'-OH terminus of the probe to increase the sensitivity of the assay. Poly-dT is the preferred pyrimidine nucleotide. Polypyrimidine nucleo¬ tide "tail" sequences may be thus generated by cloning the probe directly into a polypyrimidine sequence. The probe is inserted into a cloning vector adjacent to a poly-dT sequence. Useful cloning vectors for this purpose include single-stranded phage vectors such as M-13 which are able to secrete phage particles containing poly dT- tailed DNA probe molecules in single-stranded form, ready for use.

The tail sequence may also be generated by enzymati- cally polymerizing pyrimidine nucleotides onto the 3'-OH terminal end of the probe with terminal deoxynucleotide transferase. A typical tail prepared in this manner comprises between about 600 and about 2000 nucleotides. •The creation of polypyrimidine nucleotide tails on the probe molecules results in increased sensitivity in the assay technique. Probes with poly-dT tails constitute a larger target for generation of UV-photoproducts. More¬ over, after the probe molecules have been hybridized to form duplexes with complementary DNA, the single-stranded poly-dT tails remain unhybridized, and protrude from the duplex molecules. The photoproducts contained in the tail are.easily accessible to anti-UV-DNA antibodies, and provide improved antibody binding, making the test system more sensitive.

The present process of photoimmune detection of hybrid molecules is illustrated diagramatically in Figure 2:

(1) Preparation of probe-DNA: The DNA containing the desired probe sequences are cleaved with restriction enzymes or fragmented mechanically to create more ends. The probe-DNA is then denatured at 100°C for five minutes, followed by cooling on ice (Fig. 2-1).

(2) (Optional) The probe-DNA is treated with terminal transferase and dTTP to generate a poly-dT-tail sequence at

the 3*-OH end of the molecule (Fig. 2-2). Alternatively, the tail is put into position by cloning the probe directly into a poly-dT-sequence.

(3) The probe-DNA is labelled by UV-irradiation (254 nm) which leads to the formation of UV-photoproducts that are recognizable by antibodies (Fig. 2-3).

(4) DNA-DNA- (or DNA-RNA-) hybridization: The labelled probe is incubated with a membrane containing samples of single-stranded DNA (or RNA), followed by washing (Fig. 2-4).

(5) The membrane with the hybridized probe-DNA is incubated with anti-UV-DNA-antibody (Fig. 2-5). Excess antibody is washed off.

(6) The complex thus formed is incubated with secondary antibody to the anti-UV-DNA-antibody, e.g. swine- anti-rabbit IgG, linked to horseradish-peroxidase (Fig. . 2-6). Excess antibody is washed off.

(7) The membrane _^ is developed by means of hydrogen peroxide and diaminobenzidine (Fig. 2-7). in addition to identi ication of specific nucleic acid sequences using probe molecules, the present invention may also be used to detect total nucleic acid. A sample purportedly containing nucleic acid is irradiated with ultraviolet light to form UV-photoproducts in the nucleic acid. The sample is then contacted with antibody to UV- irradiated nucleic acid. A signalling means associated with.the antibody signals the binding of antibody, thereby indicating the presence of nucleic acid in the sample. The various signalling means employed to detect the presence of antibody binding to probe/nucleic acid hybrids may also be used in the present method for detecting total nucleic acid. The system offers an extremely sensitive method for measuring low nucleic acid concentrations in solution as well as in gels.

According to one embodiment, a sample to be analyzed is irradiated by UV-light. The DNA in the sample is denatured and attached to a filter membrane through a

dot-blot apparatus or from an agarose-gel by an electro- blotting technique. The DNA is detected directly by binding of anti-UV-DNA antibody.

A quantitative analysis for DNA includes irradiating the sample containing DNA with ultraviolet light to form photoproducts. The DNA in the sample is then denatured and contacted with anti-UV-DNA antibody labelled with a signalling means. The signal level thereby generated is compared with the signal level from a DNA sample having a similar G-C/A-T ratio containing a known concentration of DNA which has been subject to identical irradiation, denaturation and antibody treatment as the test sample DNA. This method, which can detect nucleic acids down to the picogram range, does not involve DNA-probes or other detection principles that depend upon incorporation of radioisotopes or modified nucleotides.

Detection and/or quantification of total nucleic acid or total DNA according to the present method may be used in recombinant DNA technology, such as in cDNA cloning, where the amount of genetic material to be assayed is often too small for conventional agarose/ ethidium bromide visualization. The method may also be used to monitor the level of nucleic acid during the production of vaccines, hormones, monoclonal antibodies, and other pharmacological substances produced for _in vivo administration. There exists a great need for demonstrating the absence of nucleic acid in such preparations, owing to the possible presence of oncogenes in contaminating nucleic acid. The present photoimmune detection system offers an easy, inexpensive and sensitive method for achieving this goal.

A kit for quantifying total nucleic acid in a sample includes a supply of anti-nucleic acid antibody, a panel of nucleic acid-containing samples of known nucleic acid concen¬ tration as standards, signalling means for indicating the binding of anti-nucleic acid antibody, and a means for com¬ paring the level of signal generated by nucleic acid in the test sample with the levels of signal generated by the

various members of the standard panel. The signalling means may suitably comprise any of the various signalling means discussed above, for example, an enzyme such as horseradish-peroxidase conjugated to a swine-anti- rabbit IgG, and substrates for the enzyme providing a color producing reaction. The panel of standards may be distributed in wells surrounding the test sample on a nucleic acid- binding support. The nucleic acid in the sample is quanti¬ fied by matching the color intensity of the developed sample with the appropriate standard.

The present method for detecting specific nucleic acid may likewise be adapted to kit form. Such a kit may contain a supply of UV-labeLled nucleic acid probe molecules selected so as to hybridize with the nucleic acid to be detected. The kit further contains a means for contacting the suspect sample with the probe molecules and a supply of anti-UV-nucleic acid antibodies which bind to the UV- labelled probe. The kit contains a signalling means for indicating the binding of anti-UV-nucleic acid to the probe. The contacting means may comprise a support such as nitrocellulose paper onto which the sample nucleic acid is deposited.

The present method for photoimmune detection of specific nucleic acid may be used to demonstrate the presence of particular bacteria or other organisms; to determine. the presence of resistance and/or virulence- determinants in micro-organisms; to diagnose genetic disease; to perform chromosome karyotype analyses; and to perform gene diagnostic methods such as oncogene analysis, tissue-typing or determination of predisposition to dis¬ ease. Each of the aforesaid procedures involves the detection and visualization of specific nucleotide sequences in genetic material under study.

The present method of photoimmune detection of nucleic acids has the following noteworthy advantages over detecting means utilizing radioisotopes:

(a) It is easy to label probes by UV irradia¬ tion. UV-lamps are standard equipment in most laboratories in the form of germicidal lamps. Even short probes, for instance, probes formed from synthetic DNA, may be utilized according to the present invention by first attaching a polypyrimidine nucleotide sequence to the 3'-OH probe terminus. Such terminal sequences may contain many photo¬ products which can be easily recognized by antibodies.

(b) No expensive chemicals or costly radioactive nucleotides are required for probe labelling.

(c) The UV-labelled probe molecules are stable. It is believed that such probe molecules may be stored for years after labelling. It is thus possible to generate and store extensive robe libraries for use according to the method of the present invention.

(d) The problems and dangers connected with handling of radioactive isotopes are avoided.

(e) The method is quicker than existing radio-

> isotope-labelling methods, as no time is spent in developing audioradiographic films, or in scintillograp ic counting.

Photoimmune detection of nucleic acid has the following advantages over detection means utilizing biotinylated dUTP:

(a) Photoimmune detection, which relies on the generation of antigenic determinants _in_ situ with UV-light, is much simpler and quicker than assay methods using nick- translation for the incorporation of biotinylated dUTP.

(b) The present method s more economical, as the use of expensive chemically-synthesized biotinylated dUTP is avoided.

(c) Labelling by nick-translation is unsuitable for extremely short probes, and is useless for probes formed from single-stranded nucleic acids. The present photoimmune detection method may be employed for all types of probes. Short probes may be used by first attaching a polypyrimidine nucleotide sequence to the 3*-OH end of the probe.

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(d) The present method is at least as sensitive in detecting nucleotide sequences as the nick-translation biotin-labelling method.

(e) The present method may be used as a highly sensitive technique to detect the presence of total nucleic acid in a sample, whereas the prior art methods may be used to detect specific nucleic acid sequences only.

The present invention is illustrated through the following non-limiting examples.

Example 1 Photoimmune Detection of DNA Probes Hybridized To Sample DNA Plasmid pPHR20 containing a 2.1 kb KpnI-fragment of ribosomal DNA isolated from the slime mold Physarum polycephalum was used as a probe. The probe DNA was labelled by UV-irradiation (254 n , approximately 4 kJ/m ² ) , linearized by cleavage with the restriction enzyme PstI and denatured by heat treatment (95°C, 5 min.), followed by a quick chill in ice water to keep the DNA single-stranded.

The DNA samples to be tested were generated as follows. The plasmid pPHR20 was cleaved into four fragments of the following amounts with the restriction enzyme PstI: (1) 250 ng; (2) 680 ng; (3) 68 ng; and (4) 6.8 ng. A fifth sample comprised a 343 ng fragment of Kpn I-cleaved pPHR20. A sixth sample comprised 100 ng of the plasmid pPYAlOl. The fragments were separated by electrophoresis on a 0.7% agarose gel. The gel was then soaked in alkaline solution (0.2 M NaOH, 5 in.), neutralized in electrophoresis buffer, pH 7.5, and transferred onto a New England Nuclear "Gene Screen" membrane by electroblotting (electrophoretic transfer of the DNA fragments out of the gel plane onto the attached membrane filter giving an "offprint" of the DNA fragment pattern on the membrane.) The membrane was then baked in a vacuum oven for 2 hrs at 80°C. The UV- irradiated single-stranded DNA-probe was hybridized to the baked membrane at 42°C for 18 hours in 50% formamide accordin to Maniatis, et al. , Molecular Cloning, p. 324-27. Following

hybridization, nonspecific antibody binding sites were saturated by incubating the membrane with a 10% solution of swine serum in tris-buffered saline, TBS (20 M Tris-HCl, pH 7.6, 0.5 M NaCl) at 20°C for one hour with gentle shaking. The primary antibody, rabbit-anti-UV-DNA antibody, in a 10% swine serum dilution, was subsequently allowed to attach to the membrane-bound DNA by incubation for 30 min at 20°C. After washing (3x5 min. in TBS, 0.05% Tween 20) the secondary antibody, biotinylated goat-anti-rabbit IgG was allowed to bind, followed by incubation with streptavidin and biotinylated alkaline phosphatase (protocol by the supplier - Bethesda Research Laboratory) .

The results shown in Fig. 3 illustrate the sensitivity of the detection system. The amount of DNA loaded in lanes 1, 2 and 5 giving bands containing DNA in the 100 ng range (which is the normal level for ethidiu bromide staining), appears as heavily overloaded when probed with the photoimmune detection system. However, DNA bands in the 0.1-10 ng range (lanes 3 and 4) are more appropriate for qualitative identification.

Examole 2

Photoimmune Detection With DNA- Probes Extended With PolydT-tails Photoimmune detection of specific DNA from the bacteriophage Hyl7 was demonstrated using DNA-probe mole¬ cules with appropriate poly-dT tail sequences. Hyl7 is an artificially-made bacteriophage consisting of a defined chromosome segment from each of the bacteriophages P2 and P4. Various quantities (8-1000 ng) of the DNA-samples were applied to a membrane (Gene-Screen Hybridization Transfer Membrane NEF-927, New England Nuclear) by means of a icrofiltration apparatus (Bio-Dot Microfiltration Apparatus, 3io-Rad Labora¬ tories) in accordance with the instruction manual for the "Gen Screen"membrane (New England Nuclear Catalogue No. NEF 872, Gene-Screen: Hybridization Transfer Membrane-Instruction Manual) .

Single-stranded ("ss") and double-stranded ("ds") DNA from phage lambda and single-stranded calf thymus DNA ("C.th.DNA" ) , were used as negative controls.

A probe which consisted of Hyl7-DNA cleaved with the restriction enzyme Hindlll was used for the hybridization- reaction. The fragments were provided with tails, on average of 1900 thymine-bases, by terminal transferase. Prior to hybridization, the probe molecules were irradiated with 2500 J/m ² UV-light (254 nm) . The hybridization was performed in acordance with "Method III" in the afore¬ mentioned instruction manual for the "Gene-Screen"membrane.

After hybridization, the membrane was washed and incubated with 10% of normal swine serum in tris-buffered saline (TBS = O.02 M Tis-HCl, pH 7.6, with 0.5 M NaCl added) with gentle shaking at room temperatuer for one hour. This step was followed by incubation for 30 min. at 20°C with a suitable dilution of anti-UV-DNA-antibody in TBS which contained 10% swine serum. After washing (3x5 min.) with TBS containing 0.05% of Tween 20, the membrane was incubated for 30 min. with swine-anti-rabbit-antibody which was linked to horseradish-peroxidase (DAKOPATTS A/S, DENMARKO) and diluted 1:200 in TBS with 10% or normal swine serum. The membrane was washed again (3 5 min.) and incubated with diaminobenzidine and H2O2 according to Adams, J.C., Histochem Cytochem. , 2_9: 775 (1981). The results appear in Figure 4.

From Fig. 4 it is clear that only the blots containing Hyl7-DNA, which is homologous to the probe-DNA, are stained. The specificity of the reaction is demonstrated by the absence of a color reaction for ss and ds lambda-DNA, and for C.th-DNA. The stain furthest to the right contains only 8 ng of DNA, and thus provides an indication of the sensitivity of the technique. By increasing the sensitivity at the detection stage, it is beleived possible to detect DNA in the pg-range.

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Example 3 Photoimmune Detection of DNA Compared To Ethidiu Bromide-Staining The sensitivity of the present photoimmune assay for detecting DNA was compared to the sensitivity of the conventional ethidium bromide-staining technique according to the following procedure. DNA from P2, P4, lambda and pBR 322 were cleaved with the restriction enzymes Hpal, EcoRI, Bglll and Hindlll, and separated by agarose-gel electrophoresis (0.7% agarose in the gel; 40 mM tris-acetate buffer with 1 mM EDTA, pH 8) . The gel was stained with ethidium bromide (0.5 microgram/ml and visualized over UV-light from a transilluminator (Fotodyne, 300 nm) which simultaneously generated UV-photoproducts. The ethidium bromide-stained gel is shown in Fig. 5a. The DNA in the gel was denatured in 0.2 N NaOH and transferred to a "Gene- Screen" membrane by electroblotting according to the instruction manual for the "Gene-Screen". The membrane was subsequently incubated with swine serum and stained according to the method described in Example 2. The result is shown in Fig. 5b.

A comparison of Figures 5a and 5b reveals that photoimmune detection of DNA according to the present invention (5b) works well, and is more sensitive than conventional ethidium bromide-staining (5a). For instance, in lane 2 from the top, showing P4 DNA fragmented with Hpal, several of the faint bands which can be seen in Fig. 5a are significantly stronger in Fig. 5b, thereby indicat¬ ing the increased sensitivity of the photoimmune detection over ethidium bromide-staining.

The present invention may be embodied in other specifi forms without departing from the spirit or essential attri¬ butes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specifica¬ tion, as indicating the scope of the invention.