Targeting in linear DNA duplexes with two complementary probe strands for hybrid stability.

A new in vitro hybridization reaction targets two short complementary RecA protein-coated DNA probes to homologous sequences at any position in a linear duplex DNA molecule. Stable hybrids are obtained after RecA protein removal when both complementary probe strands are present in a four-stranded hybrid, but not when one probe strand is present in a three-stranded hybrid. In four-stranded hybrids with one probe strand biotinylated and the other radiolabelled, the deproteinized hybrids can be isolated and detected by affinity capture on streptavidin-coated magnetic beads. RecA-mediated targeting of complementary biotinylated DNA probe strands allows the affinity capture of 48.5-kilobase duplex lambda genomic DNA. These reactions provide a means of isolating any desired duplex gene or chromosomal DNA fragment.

Homologous DNA targeting with RecA protein-coated short DNA probes and electron microscope mapping on linear duplex molecules.

We demonstrate that RecA protein-coated, short single-stranded DNA probes paired with a specific homologous DNA sequence in a linear duplex target molecule and accurately targeted the selected DNA sequence. RecA protein-coated complementary ssDNA probes were reacted with linear duplexes, and the homologously paired molecules were observed by electron microscopy. The sites of interaction between the RecA protein-coated DNA probes and the uncoated duplex DNA targets were directly visible on individual target DNA molecules by high-resolution darkfield electron microscopy, without chemical fixation or sample shadowing. The efficiency and specificity of pairing were verified with 446 and 222 base single-stranded DNA probes that shared no homology with one another, and several linear duplex target DNAs with their respective probe homology sites at different locations with respect to the ends of the double-stranded DNA molecules. Measurements of the position of RecA protein-coated probes paired to individual target molecules, observed at high magnification, showed that DNA probes specifically paired at their corresponding homologous target sequences. This RecA protein-mediated DNA mapping method allows homologous sequence positioning and gene mapping on individual double-stranded DNA molecules. Targeting reactions in which two different probe/target sites were 900 bases apart on a single duplex target molecule allowed both sites to be mapped in the same targeting reaction; although targets displaying both probes simultaneously were seen much less frequently than expected. The possible torsional or mechanistic constraints related to these reactions are briefly discussed.

Cytoplasmic microinjection of immunoglobulin Gs recognizing RNA helices inhibits human cell growth.

We report here that nucleolar and cytoplasmic RNA in mammalian cells is recognized specifically by both experimentally induced monoclonal IgG unique for left-handed Z-RNA and by autoimmune mouse monoclonal IgG specific for ribosomal RNA. Nucleolar Z-RNA synthesis, like nucleolar ribosomal RNA synthesis, is inhibited by actinomycin D treatment and dimethylsulfoxide-induced differentiation. Immune anti-Z-RNA IgGs microinjected into living nuclei bind nucleolar RNA, and these complexes appear to be removed from the nucleus within minutes. Cytoplasmically microinjected monoclonal or polyclonal anti-Z-RNA IgGs specifically bind cytoplasmic RNA and inhibit cell multiplication. Microinjection of antibodies directed against double-stranded RNAs. Elevated ionic conditions, which in energy-minimized models can cause the walls of the groove in Z-RNA (but not Z-DNA) to approach each other and close, also prevent antibody binding to specific synthetic or cellular Z-RNA determinants. Our antibodies binding unique Z-RNA structures probably recognize antigens determined by the exposed 2'-OH ribose sugar-phosphate groups.

Immunoglobulin recognition of synthetic and natural left-handed Z DNA conformations and sequences.

The relative immunogenicities of the poly[d(G-C)] and poly[d(A-C).d(G-T)] families of helices have been determined. The specificities of the resultant immunoglobulins have been characterized for recognition of different synthetic and natural left-handed sequences and conformations. Certain modifications of poly[d(G-C)] in the sugar-phosphate backbone and cytosine C-5 potentiate the right(R)-to-left(L) (B----Z) transition under physiological conditions. The resulting polynucleotides, poly[d(G-SC)], poly[d(G-io5C)], poly[d(G-br5C)] and poly[d(G-m5C)], are also highly immunogenic. In contrast, DNAs incapable of assuming the left-handed conformation under physiological salt concentrations are weakly or non-immunogenic. These include unmodified poly[d(G-C)] as well as members of the poly[d(A-C).d(G-T)] family of sequences bearing pyrimidine C-5 substitutions (methyl, bromo, iodo). These polynucleotides undergo the R----L isomerization under more stringent ionic and thermal conditions. The specificities of purified polyclonal and monoclonal anti-Z DNA immunoglobulins (IgG) were measured by binding to radiolabeled polynucleotides, by electrophoretic analysis of IgG bound to covalent closed circular DNAs, and by immunofluorescent staining of polytene chromosomes. The salt-induced left-handed forms of poly[d(G-C)] and its derivatives (including the cytidine C-5 methyl, bromo, iodo, and N-5 aza substituted polynucleotides) and of the modified poly[d(A-C).d(G-T)] polymers are bound to varying degrees by different antibodies. The patterns of substrate recognition demonstrate the existence of several antigenic domains in left-handed DNAs, including the helix convex surface and the sugar-phosphate backbone. Substitutions in these regions can produce enhancing (required substitutions), neutral, or inhibitory effects on subsequent IgG binding. Additionally, certain modifications of either the convex surface of Z DNA at the C-5 position of cytidine (i.e. a methyl group) or of the backbone (i.e. phosphorothioate substitution) can lead to polymorphic left-handed conformations that are compatible with antibody binding when present individually but not in combination. The recognition patterns exhibited with DNA substrates from the two DNA families indicate that some, but not all, IgGs show specificity for different nucleotide sequences. The anti-Z DNA IgGs were used to probe for specific left-handed Z DNA determinants on plasmid (e.g. pBR322) or viral (e.g. simian virus 40 (SV40] DNAs and on the acid-fixed polytene chromosomes of dipteran larvae.(ABSTRACT TRUNCATED AT 400 WORDS)

Duration time of a one-dimensional random walk as a function of the energies of the intermediate states: application for dissociation and relaxation processes in DNA hybrids.

Kinetic parameters of macromolecular systems are important for their function in vitro and in vivo. These parameters describe how fast the system dissociates (the characteristic dissociation time), and how fast the system reaches equilibrium (characteristic relaxation time). For many macromolecular systems, the transitions within the systems are described as a random walk through a number of states with various free energies. The rate of transition between two given states within the system is characterized by the average time which passes between starting the movement from one state, and reaching the other state. This time is referred to as the mean first-passage time between two given states. The characteristic dissociation and relaxation times of the system depend on the first-passages times between the states within the system. Here, for a one-dimensional random walk we derived an equation, which connects the mean first-passage time between two states with the free energies of the states within the system. We also derived the general equation, which is not restricted to one-dimensional systems, connecting the relaxation time of the system with the first-passage times between states. The application of these equations to DNA branch migration, DNA structural transitions and other processes is discussed.

Theoretical analysis of DNA branch migration in the presence of a slow reversible initiation step.

Branched DNA structures include several DNA regions connected by three- or four-way DNA junctions. Branched DNAs can be intermediates in DNA replication and recombination in living organisms and in sequence-specific DNA targeting in vitro. Branched DNA structures are usually metastable and irreversibly dissociate to non-branched products via a DNA strand exchange process commonly known as DNA branch migration. The key parameter in the DNA dissociation process is its characteristic time, which depends on the length of the dissociating DNA structure. Here, we predict that the presence of a slow reversible initiation step, which precedes DNA branch migration, can alter, to almost linear dependence, the "classic" quadratic dependence of the dissociation time on the length of the dissociating DNA structure. This prediction can be applied to dissociation of Y-like DNA structures and double D-loop DNA hybrids, which are DNA structures similar to replication bubbles. In addition, the slow initiation step can increase the effect of DNA sequence heterologies within the structure on its kinetic stability. Applications of our analysis for genetic manipulations with branched DNA structures are discussed.

Recognition of Z-RNA and Z-DNA determinants by polyamines in solution: experimental and theoretical studies.

Protonated polyamines are among the most efficient cations that induce the left-handed Z-form in certain polynucleotides. It is not known, however, whether these cations bind to specific sites on Z-sequences in solution. We have studied potential polyamine binding sites by measuring the effects of polyamines on the binding of purified immunoglobulins (IgGs) to different regions of the Z-helix and by molecular mechanics modeling. The specific binding of anti-Z-DNA and anti-Z-RNA IgGs to Z-helices was studied as a function of spermidine or spermine concentration. The effect of polyamines on the antibody-nucleic acid interaction was different for IgGs with different specificities for various determinants on the Z-helix. Polyamines inhibit the binding of certain anti-Z IgGs directed against specific sites probably at or near the interface between the major convex surface and the phosphate backbone, most likely by competing with the antibody binding site(s). In contrast, polyamines have no effect on other anti-Z IgGs directed against sites determined by the phosphate backbone. Furthermore, these cations can enhance the binding of anti-Z IgG directed against bulky groups at the C-5 position on the major convex surface of the helix; the enhancement may be related to charge neutralization. Under these conditions, no direct binding of antibodies with polyamines was observed. These data suggest the existence of a specific binding site(s) for polyamines on both Z-DNA and Z-RNA in solution. These binding sites have some similarity to those observed in oligonucleotide crystals by Quigley (in "Molecular Structure and Biological Activity," J.F. Griffin and W.L. Duax, eds., Elsevier, Amsterdam (1982), pp. 317-331). The experimental evidence for specific spermine binding sites on the helical surface was supported by molecular mechanics modeling of the interaction of spermine with the major groove of (dG-dC)5.(dG-dC)5 in both the Z- and B-forms. The crystal coordinates of spermine-containing oligonucleotides in both the B- and Z-forms were used as the starting points for modeling studies. The potential energy of spermine bound to the major convex surface of the Z-form was much less favorable than that of spermine bound to the major groove of the B-form. In the presence of sodium ions, however, the Z-form-spermine complexes were favored over the B-form. Thus, both theoretical and experimental studies indicate that polyamines can specifically recognize Z-helical determinants in solution as well as in crystals.

Interactions of anti-poly[d(G-br5C)] IgG with synthetic, viral and cellular Z DNAs.

Enzymatically synthesized poly[d(G-br5C)] was used to prepare specific polyclonal and monoclonal anti-Z DNA IgGs. The binding specificities of these antibodies were characterized using left-handed polynucleotides with the sequences d(G-x5C)n and d(A-x5C)n.d(G-T)n (mean = aza, methyl, bromo, or iodo). Polyclonal anti-poly[d(G-br5C)] IgG binds the convex surface of the Z helix as evidenced by the strong requirement for a methyl or halogen group at the C5 position of cytosine. Little or no anti-poly[d(G-br5C)] IgG binding occurs to left-handed DNAs carrying a phosphorothioate substitution in the dGpdC bond or an N-5 aza substitution in the cytosine ring. Anti-poly[d(G-br5C)] IgG can stabilize transient Z DNA structures in both polymer families, thereby displacing the equilibrium in solution between the right-and left-handed DNA conformations. Anti-poly[d(G-br5C)] IgG binding sites are found in all tested covalently closed circular natural DNAs (Form I) at their extracted negative superhelical densities, but not in any of the corresponding relaxed Form II or linear Form III DNAs. Binding of anti-poly[d(G-br5-C)] IgG leads to a reduction in the electrophoretic mobility of Form I DNA (e.g. SV40, phi X174, or pBR322) and to the formation of dimers comprised of the bivalent antibody and two supercoiled Form I DNA molecules. The dimers are converted to monomers by DTT treatment. The formation of IgG-DNA complexes is dependent on external conditions (ionic strength, temperature), the properties of the DNA (torsional stress, sequence), and the immunoglobulin (specificity, valency, and concentration). Higher order oligomeric species, indicative of two or more left-handed segments per DNA molecule are formed in reactions of anti-poly[d(G-br5C)] IgG with M13 RF I DNA but not with SV40, pBR322, or phi X174 DNAs. However, oligomers of the latter are generated with other anti-Z DNA IgGs having a broader spectrum of anti-Z DNA reactivity. Conditions which destabilize natural Z sequences in deproteinized supercoiled genomes are: monovalent salt concentrations at or above the 'physiological' range, high temperature, and topological relaxation with DNA gyrase (in the absence of ATP) or with type I topoisomerases. DNA gyrase (plus ATP) catalyses an increase in DNA negative superhelical density which leads to greater anti-Z DNA IgG binding, indicating the formation of additional left-handed regions. Polytene chromosomes of insect larvae bind anti-poly[d(G-br5C)] IgG specifically and stably at Z DNA sites. The distribution of this IgG binding differs in certain regions from that displayed by anti-Z DNA IgG probes with other sequence specificities.(ABSTRACT TRUNCATED AT 400 WORDS)

Natural occurrence of left-handed (Z) regions in PM2 DNA.

Bacteriophage PM2 DNA, a ccc genome of high apparent superhelical density, contains left-handed (Z) regions as detected by competitive radioimmunoassay, agarose gel electrophoresis of DNA: antibody complexes and immunoelectron microscopy. The latter technique, in conjunction with partial blockage of restriction endonuclease sites by bound antibody, was used to map the left-handed regions along the DNA molecule. A cluster of four to five antibody molecules (approximately 25% of bound antibody) was located within map units 0.05-0.18 of the single Hpa II restriction site. Sequence analysis of part of this region showed the presence of several areas of high alternating purine-pyrimidine content. A strong correlation is observed between alternating pyrimidine-purine tracts of significant length and antibody binding sites.

Left-handed DNA: from synthetic polymers to chromosomes.

The interconversions between right-handed (R) and left-handed (L) helical conformations of DNA have been assessed by spectroscopic, electrophoretic, immunochemical, and enzymatic techniques. We have screened salt and solvent conditions which facilitate these transitions, as well as certain chemical modifications of the bases and backbone of defined synthetic polynucleotides. These include major and minor groove substituents as well as phosphorothioate analogues of selected phosphodiester bonds. We have established: R-L transitions in poly[d(G-C)] with iodo, bromo, methyl, and aza substitutions at the C5 position of cytosine, or phosphorothioate modification of the dGpC linkage. R-L transitions in the [d(A-C).d(G-T)]n sequence family using polymers modified as in the case of poly[d(G-C)]. The isomerizations are highly salt and temperature dependent. a possible L form of poly[d(A-T)] substituted with 2-amino adenine. the immunogenicities of constitutive and facultative Z-DNAs. the recognition specificities of different anti-Z-DNA IgGs for the spectrum of available polynucleotide probes. Some IgGs are sequence-specific. stabilization by IgG of otherwise transient left-handed conformations. anti-Z-DNA IgG binding to acid-fixed polytene chromosomes from the Diptera Drosophila, Chironomus, and Glyptotendipes. Laser scanning microscopy shows a maximal binding of 1 IgG per 3000-15,000 basepairs in acid fixed preparations. anti-Z-DNA IgG binding to negatively supercoiled plasmid, viral, phage, and recombinant closed circular DNAs. transcription from Z and Z* (associated) left-handed templates. From these and other results we propose that Z*-DNA may have important structural-functional roles in the cell.

Spontaneous mutation of RNA tumour viruses.

There are 2 categories of spontaneously occurring avian and mammalian RNA tumour virus mutants: conditional and non-conditional. 1) Conditional mutants are able to replicate in or transform cells only under certain physiological conditions or in certain cells. RNA tumour virus temperature-sensitive mutants, focus-morphology mutants, and host range mutants are spontaneously formed. Some of these conditional mutants probably arise by point mutations in the viral genome. 2) Non-conditional mutants have genetic lesions that render them inactive under all conditions. There are non-conditional spontaneous RNA tumour virus mutants that are missing either the virion envelope glycoprotein or both the envelope glycoprotein and the virion DNA polymerase. These mutants cannot replicate or transform cells. Other spontaneous non-conditional mutants can replicate but are defective in their ability to transform fibroblastoid cells. These spontaneous transformation-defective mutants can have deletions in 10-20% of the genomic RNA. Conditional mutants with an altered host range occur at a high rate of approximately 1 mutation/50 infected cell generations during DNA-to-DNA information transfer. This type of conditional mutation requires cell replication but does not occur frequently either during the original synthesis of viral DNA (RNA-to-DNA information transfer) or during the transcription of progeny viral RNA from the (RNA-to-DNA information transfer) or during the transcription of progeny viral RNA from the DNA (DNA-to-RNA information transfer). Temperature-sensitive and focus-morphology mutants also have a high rate of spontaneous formation. Non-conditional mutants missing the viral envelope glycoprotein, DNA polymerase, or transformation gene, also appear to be spontaneously formed at a high rate. Normal avian and mammalian cells contain RNA tumour virus-related genes in their DNA. It is hypothesized that these endogenous RNA tumour virus-related genes in normal cells also have a high rate of spontaneous mutation and are involved in neoplastic processes.

High spontaneous mutation rate of an avian sarcoma virus.

Three genetically distinct types of chicken sarcoma virus Bratislava 77 (B77 virus) differing in their ability to infect duck cells were identified. B77 virus type I does not infect duck cells; B77 virus type II has a low efficiency of infection of duck cells; and B77 virus type III has a high efficiency of infection of duck cells. B77 viruses type I and III are produced by spontaneous mutation during the growth of B77 virus type II in chicken cells. The spontaneous mutation of B77 virus type II TO B77 virus type III occurs with a high rate (approximately 1 mutation per 50 infected cell generations), requires cell replication, and neither occurs during the synthesis of viral DNA on an RNA template nor during the transcription of progeny viral RNA from the provirus. The rate of spontaneous mutation of B77 virus type II to B77 virus type I is greater than the rate of spontaneous mutation of B77 virus type II to B77 virus type III.

Unique poly(dA).poly(dT) B'-conformation in cellular and synthetic DNAs.

Poly(dA).poly(dT), but not B-form DNA, is specifically recognized by experimentally induced anti-kinetoplast or anti-poly(dA).poly(dT) immunoglobulins. Antibody binding is completely competed by poly(dA).poly(dT) and poly(dA).poly(dU) but not by other single- or double-stranded DNA sequences in a right-handed B-form. Antibody interaction with poly(dA).poly(dT) depends on immunoglobulin concentration, incubation time and temperature, and is sensitive to elevated ionic strengths. Similar conformations, for example, (dA)4-6 X (dT)4-6, in the kinetoplast DNA of the parasite Leishmania tarentolae are also immunogenic and induce specific anti-poly(dA).poly(dT) antibodies. These antibody probes specifically recognize nuclear and kinetoplast DNA in fixed flagellated kinetoplastid cells as evidenced by immunofluorescence microscopy. Anti-poly(dA).poly(dT) immunofluorescence is DNase-sensitive and competed by poly(dA).poly(dT), but not other classical double-stranded B-DNAs. Thus, these unique cellular B'-DNA helices are immunogenic and structurally similar to synthetic poly(dA).poly(dT) helices in solution.

DNA hybrids stabilized by heterologies.

The double D-loop DNA hybrid contains four DNA strands following hybridization of two RecA protein coated complementary single-stranded DNA probes with a homologous region of a double-stranded DNA target. A remarkable feature of the double D-loop DNA hybrids is their kinetic stabilities at internal sites within linear DNA targets after removal of RecA protein from hybrids. We report here that heterologous DNA inserts in one or both probe strands affect the kinetic stability of protein-free double D-loop hybrids. DNA heterologies normally distort DNA-DNA hybrids and consequently accelerate hybrid dissociation. In contrast, heterologous DNA inserts impede dissociation of double D-loops, especially when the insert sequences interact with each other by DNA base pairing. We propose a mechanism for this kinetic stabilization by heterologous DNA inserts based on the hypothesis that the main pathway of dissociation of double D-loop DNA hybrids is a DNA branch migration process involving the rotation of both probe-target duplexes in the hybrids. Heterologous DNA inserts constrain rotation of probe-target duplexes and consequently impede hybrid dissociation. Potential applications of the stabilized double D-loops for gene targeting are discussed.

The availability for recognition of normal H-2 antigens by cytotoxic T lymphocytes on a Rauscher-virus-transformed cell, RBL-5A.

The availability for recognition of cell surface H-2Db antigenic determinants on RBL-5A cells in comparison to normal C57BL/L cells was investigated by an in vitro immunological assay. No differences in the immunological recognition of the H-2Db antigens on RBL-5A cells compared to normal C57BL/L cells were detected. This assay system, however, readily detected changes in the H-2Kb determinant profile on target cells from the C57BL/L H-2Kb region mutant strains H(zl) and H(zl70) when compared to normal C57BL/6 mice. The result obtained with RBL-5A target cells therefore suggests that Rauscher murine leukaemia virus transformation does not induce, either genotypically or phenotypically, an immunologically recognizable alteration of the H-2Db antigen profile on this cell.

Electron microscopy of SV40 DNA cross-linked by anti-Z DNA IgG.

Electron microscopy has revealed the specific binding of bivalent anti-Z DNA immunoglobulin G (IgG) to different sites on supercoiled Form I SV40 DNA. The anti-Z IgG links together left-handed regions located within individual or on multiple SV40 DNA molecules at the superhelix density obtained upon extraction. Velocity sedimentation, electrophoresis, and electron microscopy all show that two or more Z DNA sites in the SV40 genome can be intermolecularly cross-linked with bivalent IgG into high mol. wt. complexes. The formation and stability of the intermolecular antibody-DNA complexes are dependent on DNA superhelix density, as judged by three criteria: (1) relaxed circular (Form II) DNA does not react; (2) release of torsional stress by intercalation of 0.25 microM ethidium bromide removes the antibody; and (3) linearization with specific restriction endonucleases reverses antibody binding and DNA cross-linking. Non-immune IgG does not bind to negatively supercoiled SV40 Form I DNA, nor are complexes observed in the presence of competitive synthetic polynucleotides constitutively in the left-handed Z conformation; B DNA has no effect. Using various restriction endonucleases, three major sites of anti-Z IgG binding have been mapped by electron microscopy to the 300-bp region containing nucleotide sequences controlling SV40 gene expression. A limited number of minor sites may also exist (at the extracted superhelix density).

Spontaneous mutations affecting the host range of the B77 strain of avian sarcoma virus involve type-specific changes in the virion envelope antigen.

Previously it was shown that the host-range gene of the Bratislava strain of avain sarcoma virus (B77 virus) spontaneously mutates with a very high rate. The wild-type B77 virus called B77 virus-II, mutates either to virus that efficiently infects duck cells (B77 virus-III) or to virus that does not mutate to the ability to infect duck cells (B77 virus-I) (Zarling and Temin, 1976). No significant differences in either the virion envelope glycoproteins or other major virion proteins were detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. However, pseudotypes of B77 virus-I with proteins of a transformation-defective mutant of B77 virus-III formed foci efficiently in duck cells. An alteration in the envelope protein of B77 virus-I was demonstrated by experiments in which B77 firus-I was fused into duck cells with UV-irradiated Sendai virus and formed foci. Neutralization experiments further demonstrated that B77 virus host-range mutants have altered type-specific envelope antigens. Thus, the spontaneous mutations in the host-range gene of B77 virus involve changes in the type-specific virion envelope antigen.

Cytoplasmic Z-RNA.

Specific immunochemical probes for Z-RNA were generated and characterized to search for possible Z-RNA-like double helices in cells. Z-RNA was detected in the cytoplasm of fixed protozoan cells by immunofluorescence microscopy using these anti-Z-RNA IgGs. In contrast, autoimmune or experimentally elicited anti-DNA antibodies, specifically reactive with B-DNA or Z-DNA, stained the nuclei. Pre-or nonimmune IgGs did not bind to the cells. RNase A or T1 digestion eliminated anti-Z-RNA IgG binding to cytoplasmic determinants; however, DNase I or mung bean nuclease had no effect. Doxorubicin and ethidium bromide prevented anti-Z-RNA antibody binding; however, actinomycin D, which does not bind double-stranded RNA, did not. Anti-Z-RNA immunofluorescence was specifically blocked in competition assays by synthetic Z-RNA but not Z-DNA, A-RNA, or single-stranded RNAs. Thus, some cytoplasmic sequences in fixed cells exist in the left-handed Z-RNA conformation.

Concordance of experimentally mapped or predicted Z-DNA sites with positions of selected alternating purine-pyrimidine tracts.

The recent electronmicroscopic and biochemical mapping of Z-DNA sites in phi X174, SV40, pBR322 and PM2 DNAs has been used to determine two sets of criteria for identification of potential Z-DNA sequences in natural DNA genomes. The prediction of potential Z-DNA tracts and corresponding statistical analysis of their occurrence have been made on a sample of 14 DNA genomes. Alternating purine and pyrimidine tracts longer than 5 base pairs in length and their clusters (quasi alternating fragments) in the 14 genomes studied are under-represented compared to the expectation from corresponding random sequences. The fragments [d(G X C)]n and [d(C X G)]n (n greater than or equal to 3) in general do not occur in circular DNA genomes and are under-represented in the linear DNAs of phages lambda and T7, whereas in linear genomes of adenoviruses they are strongly over-represented. With minor exceptions, potential Z-DNA sites are also under-represented compared to random sequences. In the 14 genomes studied, predicted Z-DNA tracts occur in non-coding as well as in protein coding regions. The predicted Z-DNA sites in phi X174, SV40, pBR322 and PM2 correspond well with those mapped experimentally. A complete listing together with a compact graphical representation of alternating purine-pyrimidine fragments and their Z-forming potential are presented.

Association of mouse major histocompatibility and Rauscher murine leukaemia virus envelope glycoprotein antigens on leukaemia cells and their recognition by syngeneic virus-immune-cytotoxic T-lymphocytes.

Physical association was measured between MLV gp70, the envelope glycoprotein of Rauscher murine leukaemia virus (R-MLV), and serologically defined H-2 antigens on the surface of R-MLV transformed C57BL/6 (H-2DbKb) mouse leukaemia cells (RBL-5A). Capping and patching with antisera against H-2Db caused specific co-capping and co-patching of R-MLV gp70 antigens as seen by fluorescence microscopy. Despite the physical proximity of R-MLV gp70 and H-2Db antigens, high titre alphaR-MLV gp70 sera had no effect in blocking syngeneic T-lymphocyte mediated cytolysis of RBL-2A cells whereas alphaH-2Db sera were effective.