Inhibition of RNA-Dependent DNA Polymerase of Avian Myeloblastosis Virus by Pyran Copolymer

PYRAN COPOLYMER, A KNOWN IMMUNOSTIMULATOR, WAS FOUND TO BE A POTENT INHIBITOR OF PURIFIED DNA POLYMERASE (DEOXYNUCLEOSIDETRIPHOSPHATE: DNA deoxynucleotidyltransferase; EC 2.7.7.7) isolated from avian myeloblastosis virus. Unlike other inhibitors, pyran showed unique features of inhibition. It interacts with the polymerase at a region other than the template site. The inhibitory effect was overcome only by excess enzyme and not affected by excess template. The degree of inhibition was not template specific for the templates tested: 70S RNA from avian myeloblastosis virus, synthetic hybrid poly(rA).oligo(dT)(10), synthetic copolymer poly(dA-dT), and activated calf-thymus DNA. The observed rate of inhibition by pyran was shown to vary with the different polymerases tested. Inhibition was shown with all oncornaviral polymerases and, to a lesser extent, with mammalian polymerases. However, two of the three bacterial polymerases, by contrast, showed a marked activation.     

In Vitro Synthesis of DNA Complementary to Purified Rabbit Globin mRNA

Several properties of the viral RNA-dependent DNA polymerases and of rabbit globin mRNA make it possible to consider synthesis of the globin gene in vitro. These enzymes copy an RNA template using a short sequence of complementary nucleotides as a primer. Furthermore, globin mRNA has a 3'-terminal sequence of adenylic acid residues that make it particularly suitable as a template, since oligo(dT) can be annealed to a specific site on the mRNA. This small primer could phase the DNA polymerase, possibly ensuring that replication is initiated from that end of the globin message. We have used this approach and find that purified mRNA is an efficient template for the polymerase enzyme. The reaction requires the RNA template and the four deoxyribonucleoside triphosphates, and it is markedly stimulated by the addition of oligo(dT). Consistent with the expectation that the oligo(dT) uniquely phases the polymerase at an adenine-rich region in the globin message, oligo(dG), oligo(dC), and oligo(dA) fail to serve as primers. The product has a density intermediate between that of DNA and RNA, and shifts to a lighter DNA density after treatment with base. Further, it is specifically complementary to globin mRNA and sediments slightly faster in an alkaline sucrose gradient than a DNA standard that has a molecular weight of 129,000. The data suggest that a major portion of the DNA product is a sequence of at least 500 bases, about 50 more than would be necessary to encode rabbit globin. The potential usefulness of this interesting product is discussed.     

Mechanism of ultraviolet-induced mutagenesis: Extent and fidelity of in vitro DNA synthesis on irradiated templates

The effect of UV irradiation on the extent and fidelity of DNA synthesis in vitro was studied by using homopolymers and primed single-stranded varphiX174 phage DNA as substrates. Unfractionated and fractionated cell-free extracts from Escherichia coli pol(+) and polA1 mutants as well as purified DNA polymerase I were used as sources of enzymatic activity. (DNA polymerases, as used here, refer to deoxynucleosidetriphosphate:DNA deoxynucleotidyltransferase, EC 2.7.7.7.) The extent of inhibition of DNA synthesis on UV-irradiated varphiX174 DNA suggested that pyrimidine dimers act as an absolute block for chain elongation by DNA polymerases I and III. Experiments with an irradiated poly(dC) template failed to detect incorporation of noncomplementary bases due to pyrimidine dimers. A large increase in the turnover of nucleoside triphosphates to free monophosphates during synthesis by DNA polymerase I on irradiated varphiX174 DNA has been observed. We propose that this nucleotide turnover is due to idling by DNA polymerase (i.e., incorporation and subsequent excision of nucleotides opposite UV photolesions, by the 3'-->5' "proofreading" exonuclease) thus preventing replication past pyrimidine dimers and the potentially mutagenic event that should result. In support of this hypothesis, DNA synthesis by DNA polymerase from avian myeloblastosis virus and by mammalian DNA polymerase alpha, both of which are devoid of any exonuclease activity, was found to be only partially inhibited, but not blocked, by UV irradiation of the template and accompanied by an increased incorporation of noncomplementary nucleotides. It is suggested that UV mutagenesis in bacteria requires an induced modification of the cellular DNA replication machinery, possibly an inhibition of the 3'-->5' exonuclease activity associated with DNA polymerases.     

Demonstration of Globin Messenger Sequences in Giant Nuclear Precursors of Messenger RNA of Avian Erythroblasts

Highly purified globin mRNA from ducks was copied with RNA-directed DNA polymerase from avian myeloblastosis virus into anti-messenger DNA. With excess RNA, more than 90% of this DNA annealed back to its template with a C(o)t/2 value of 7.5 x 10(-4) mol.sec. liter(-1); the melting temperature of the hybrid was 86 degrees . Giant nuclear RNA fractions with sedimentation coefficients of more than 50 S formed hybrids of almost equal stability at C(o)t/2 values of 0.05-0.42 mol.sec. liter(-1), indicating amRNA content of 0.3-1.5%. 12S RNA from the same polyribosomes and nuclear giant RNA from HeLa cells did not cross-hybridize. Although a large part of the giant RNA broke down in 99% dimethylsulfoxide gradients, RNA fractions sedimenting faster than 28S rRNA still were found to consist of up to 0.03% globin mRNA sequences. Thus, the mRNA sequences are contained in the covalent structure of giant nuclear precursors, which are termed precursor-mRNA.     

Enzymatic Synthesis of Solid Phase-Bound DNA Sequences Corresponding to Specific Mammalian Genes

With oligo(dT)-cellulose as primer, RNA-dependent DNA polymerase catalyzes the synthesis of cellulose-bound DNA that is complementary to mouse globin mRNA. The resulting cellulose-bound or solid phase complementary DNA hybridizes specifically with globin mRNA and permits the recovery of intact globin mRNA. This simple technique for the synthesis of solid phase-bound complementary DNA provides an additional and convenient method for the purification of specific genetic sequences.     

Metal-induced infidelity during DNA synthesis.

The effect of several divalent cations on the accuracy of DNA replication in vitro has been examined. Only Be2+ altered the accuracy of DNA synthesis using purified DNA polymerase (DNA nucleotidyltransferase; deoxynucleosidetriphosphate:DNA deoxynucleotidyltransferase; EC 2.7.7.7) from avian myeloblastosis virus. The Be2+-induced base substitutions occurred with all templates and with all nucleotides tested. Analysis of the product by equilibrium density centrifugation and processive hydrolysis with snake venom phosphodiesterase suggested that the noncomplementary nucleotides were present in phosphodiester linkage. Nearest neighbor studies indicated that many of the Be2+-induced errors were present as single base substitutions. The enhancement of error frequency could be duplicated by the pretreatment of the enzyme, but not the template, with Be2+. Glycerol gradient centrifugation dissociated the Be2+-DNA polymerase complex and restored the initial error frequency of the polymerase. Thus, the weak binding of a metal cation to a DNA polymerase could alter the accuracy with which that polymerase copied DNA. Beryllium is a known carcinogen. The potential use of this system as a screening technique to detect chemical mutagens and carcinogens is considered.