Mechanism of release of active alpha subunit from dimeric alpha beta avian myeloblastosis virus DNA polymerase

Reverse transcriptase from simian foamy virus serotype 1: purification and characterization

Chromatography on heparin-Sepharose, known for its affinity for nucleotide-binding polypeptides, was used to purify the viral RNA-dependent DNA polymerase (reverse transcriptase) from the core polypeptides of simian foamy virus type 1. This procedure allowed the recovery of highly purified enzyme with a high specific activity. The average molecular weight of this monomeric enzyme is 81,000 and is thus comparable to that found for other known primate retroviruses. Reverse transcriptase activity of simian foamy virus type 1 requires a ribonucleotide template as a primer or otherwise a DNA with 3'-OH ends. Other optimal conditions of activity are reviewed. Heat inactivation studies led to the concept of an enzyme with two loci, one specific for the substrate and the other for the template-primer.

Radioimmunological comparison of the DNA polymerases of avian retroviruses

125I-labeled DNA polymerases of avian myeloblastosis virus and spleen necrosis virus were used in a radioimmunological characterization of avian retrovirus DNA polymerases. It was shown that avian leukosis virus and reticuloendotheliosis virus DNA polymerases do not cross-react in radioimmunoassays. Within the avian leukosis virus species, species-specific and type-specific antigenic determinants of the DNA polymerase were defined. The previous finding of genus-specific antigenic determinants in avian myeloblastosis virus and Amherst pheasant virus DNA polymerases was confirmed and extended to members of all subgroups of avian leukosis virus. It was shown that there is little immunological variation between the DNA polymerases of the four members of the reticuloendotheliosis virus species. Particles with RNA-dependent DNA polymerase activity from the allantoic fluid of normal chicken eggs and from the medium of a goose cell culture did not compete for the antibodies directed against any of the sets of antigenic determinants defined in this study.

Structural requirements for binding of bovine tRNATrp with avian myeloblastosis virus DNA polymerase

Avian DNA polymerases use host tRNATrp as the primer for transcription. Bovine tRNATrp has been previously shown to be a biologic substitute for the avian primer. A bovine tRNATrp fragment has been identified as having a high binding affinity for the polymerase. The fragment is assigned to be 67 nucleotides, and contains most of the elements required to maintain the secondary and tertiary structure of tRNATrp.

Activation of an Mg2+-dependent DNA endonuclease of avian myeloblastosis virus alpha beta DNA polymerase by in vitro proteolytic cleavage

Partial chymotryptic digestion of purified avian myeloblastosis virus alpha beta DNA polymerase resulted in the activation of a Mg2+-dependent DNA endonuclease activity. Incubation of the polymerase-protease mixture in the presence of super-coiled DNA and Mg2+ permitted detection of the cleaved polymerase fragment possessing DNA nicking activity. Protease digestion conditions were established permitting selective cleavage of beta to alpha, which contained DNA polymerase and RNase H activity and to a family of polypeptides ranging in size from 30,000 to 34,000 daltons. These latter beta-unique fragments were purified by polyuridylate-Sepharose 4B chromatography and were shown to contain both DNA binding and DNA endonuclease activities. We have demonstrated that this group of polymerase fragments derived by chymotryptic digestion of alpha beta DNA polymerase is similar to the in vivo-isolated avian myeloblastosis virus p32pol in size, sequence, and DNA endonuclease activity.

DNA endonucleases associated with the avian myeloblastosis virus DNA polymerase

A DNA endonuclease, Endo-I, which cleaves superhelical DNAs, has been isolated from avian myeloblastosis virions stripped of their coats by mild detergent treatment. The enzyme has a broad pH optimum around 7.5-8.0 and requires Mg2+ for activity. A second endonuclease, Endo-II, with a requirement for Mn2+, also present in viral cores, copurified with avian myeloblastosis virus alpha beta DNA polymerase (reverse transcriptase, RNA-dependent DNA nucleotidyltransferase) and similarly cleaved superhelical DNAs. Heat denaturation and sodium fluoride and N-ethylmaleimide inhibition studies were carried out to demonstrate a possible relationship between the two endonucleases and the viral DNA polymerase and RNase H activities. It appears that Endo-II may be an intrinsic activity of the polymerase.

Virus-coded origin of a 32,000-dalton protein from avian retrovirus cores: structural relatedness of p32 and the beta polypeptide of the avian retrovirus DNA polymerase

A 32,000-dalton protein (p32) located in avian retrovirus cores was immunoprecipitated from [35S]methionine-labeled avian myeloblastosis virus (AMV) propagated in cultured chicken embryo fibroblast cells by an antiserum preparation (sarc III) derived from tumor-bearing hamsters injected with cloned and passaged cells from an avian sarcoma virus-induced primary hamster tumor. Since sarc III serum apparently contained antibodies only to virus-coded proteins and not to chicken cellular proteins, the immunoprecipitation of p32 from AMV by sarc III serum strongly suggested that p32 is virus coded. The origin of p32 was more definitively established by demonstrating the existence of a structural relationship between p32 and the AMV DNA polymerase. AMV p32 cross-reacted with the beta polypeptide of AMV alphabeta DNA polymerase in radioimmunoprecipitation and radioimmunoprecipitation inhibition assays, indicating that p32 and beta share common antigenic determinants. This relationship was clarified by sodium do-decyl sulfate-polyacrylamide gel electrophoretic analysis of the peptides generated by limited proteolysis of 125I-labeled AMV DNA polymerase polypeptides and of 125I-labeled AMV p32 by chymotrypsin or Staphylococcus aureus V-8 protease. The peptides which appeared during proteolytic digestion of p32 were a subset of those produced by digestion of the beta polypeptide; however, p32 had no discernible peptides in common with the alpha polypeptide. Further, all of the peptides produced by limited proteolysis of beta were present in the digests of either p32 or alpha. Our findings suggest that p32 is apparently derived by cleavage of the beta polypeptide of AMV DNA polymerase, presumably at a site near or identical to that at which alpha is generated from beta by proteolytic cleavage.

beta-Lapachone, an inhibitor of oncornavirus reverse transcriptase and eukaryotic DNA polymerase-alpha. Inhibitory effect, thiol dependence and specificity

beta-Lapachone is a naturally occuring compound that can be isolated from a number of tropical trees. It is shown to be a potent inhibitor of reverse transcriptase activity from both avian myeloblastosis virus and Rauscher murine leukaemia virus. In addition, it affects eukaryotic DNA-dependent DNA polymerase-alpha activity: 50% inhibition is reached in 60-min incubation time by about 8 micron beta-lapachone. Enzyme activity is inhibited irrespective of the purity of the enzyme used or of the amount or type of template/primer or substrate present. The inhibitory effect of the drug is only observed in the presence of dithiothreitol. The primary site of action of beta-lapachone appears to be the enzyme protein, as is also borne out by the specificity of its action. Eukaryotic DNA-dependent DNA polymerase-beta, prokaryotic DNA-dependent DNA polymerase I, several other nucleic acid polymerases and some completely unrelated enzymes are not affected. Reverse transcriptase and DNA-dependent DNA polymerase-alpha may be in someway related in possessing similarly exposed '--SH structures' in their active sites. beta-lapachone thus affords a novel means of studying such interrelationships and of further characterizing enzymes.

Reverse transcriptase of RNA tumor viruses. V. In vitro proteolysis of reverse transcriptase from avian myeloblastosis virus and isolation of a polypeptide manifesting only RNase H activity

Purified avian myeloblastosis virus reverse transcriptase contains two subunits that are structurally related. The large subunit, beta (molecular weight, 95,000), was converted in vitro by chymotrypsin into a polypeptide of molecular weight 63,000. This polypeptide was indistinguishable from the small subunit, alpha (molecular weight, 65,000), in its chromatographic behavior on the phosphocellulose column and its tryptic peptide composition. During this proteolytic conversion, a polypeptide of molecular weight 32,000 (fragment B) was obtained. It was composed of tryptic peptides unique to beta and appeared to be derived from the portion of the beta subunit that was cleaved off during the conversion of beta into alpha. Upon continued proteolysis, a smaller polypeptide of molecular weight 24,000 (fragment A) was generated. This polypeptide manifested only RNase H activity and shared common amino acid sequences with beta and alpha subunits. Fragment A did not share any amino acid sequence homology with fragment B.

Cell-free synthesis of the precursor polypeptide for avian myeloblastosis virus DNA polymerase

High molecular weight RNA (35S) isolated from avian myeloblastosis virus directs the cell-free synthesis of two prominent polypeptides of 180,000 and 76,000 molecular weight. The latter polypeptide has previously been identified as the precursor to the group-specific antigens of the virus ("gag" proteins) [Vogt, V. M., Eisenman, R. & Diggelmann, H. (1975) J. Mol. Biol. 96, 471-493]. Two-dimensional tryptic peptide analyses of the [35S]methionine-labeled peptides demonstrate that the 180,000-dalton product is a polyprotein that can account for all the peptides of the avian myeloblastosis virus DNA polymerase (DNA nucleotidyltransferase, EC 2.7.7.7) and those of the gag viral proteins. This is direct confirmation of the genomic order of the viral structural genes, placing the polymerase gene adjacent to the 5'-proximal gag gene of the virus. Furthermore, our findings suggest that the primary polymerase gene product is the beta subunit of the enzyme. These results are discussed in relation to the proposed structural gene map for the avian retraviruses and suggest a model for the in vivo processing of the viral polymerase.

In vitro transcription of the avian retrovirus genome by the alpha form of the viral RNA-directed DNA polymerase

The nature of transcription of the avian retrovirus RNA genome by the alpha form of the viral RNA-directed DNA polymerase has been investigated. Transcription was most efficient when Mn2+ was provided as the divalent metal ion. The patterns of DNA transcription using 70S RNA, 35S RNA-tRNAtrp, or 35S RNA-oligo(dT)12-18 template-primer complexes by the alpha DNA polymerase were essentially identical to those obtained using the alphabeta form. The alpha DNA polymerase appears to be deficient in the synthesis of true duplex DNA but is able to synthesize hairpin-structured DNA initiated at the 5' terminus of the viral genome on the tRNAtrp primer molecule.