Recombination occurs at a high rate in retroviral replication, and its observation requires a virion containing two different RNA molecules (heterodimeric particles). Analysis of retroviral recombinants formed after a single round of replication revealed that (i) the nonselected markers changed more frequently than expected from the rate of recombination of selected markers; (ii) the transfer of the initially synthesized minus strand strong stop DNA was either intramolecular or intermolecular; (iii) the transfer of the first synthesized plus strand strong stop DNA was always intramolecular; and (iv) there was a strong correlation between the type of transfer of the minus strand strong stop DNA and the number of template switches observed. These data suggest that retroviral recombination is ordered and occurs during the synthesis of both minus and plus strand DNA.

https://science.sciencemag.org/content/sci/250/4985/1227.full.pdf

Retroviral recombination and reverse transcription.

Summary In this essay we have considered possible relationships between errors in DNA replication and malignant progression. Theoretical and experimental studies designed to determine the energy of interaction between different nucleotides have been reviewed. The difference of the energy of interaction between complementary and noncomplementary base pairs (1 to 3 kcal) is not sufficient to account for the exactness of DNA replication that is observed in vivo or in vitro. Thus, DNA polymerases must play a role in base selection. This point has been established by the detection of polymerases that copy polynucleotide templates with few errors and the detection of others that make many errors. Different mechanisms for base selection by polymerases have been considered. Our findings that both extracts of human leukemic lymphocytes and a purified DNA polymerase from avian myeloblastosis virus have exceptionally high error rates have been considered in relationship to oncogenesis. A hypothesis is presented that relates mistakes in DNA replication, as promoted by error-prone DNA polymerases, to tumor progression. We have also considered a more general concept encompassing early cellular changes that lead to oncogenesis. The justification for this essay is that both of these hypotheses are open to immediate experimental analysis.

/pdf/errors-in-dna-replication-as-a-basis-of-malignant-changes-43nsmzgv1x.pdf

Errors in DNA Replication as a Basis of Malignant Changes

Introduction. The study of the molecular biology of retroviruses has revealed a number of features not found in other groups of viruses. In recent years, a large number of excellent reviews of the field have appeared (Table 1). In writing this article, it is not my intention to cover ground already well treated in review form. Instead, I will consider only a limited number of interrelated subjects dealing with the organization of information in tumour virus genomes and its transfer from parent to progeny. In general, I will ignore primary data and present only the inferred structures and mechanisms and point out experimental approaches that might fill in gaps in our knowledge. Retroviruses form a large and diverse group, with isolates from many different vertebrates. In most cases, our knowledge of the molecular biology of the viruses is derived from one model system, the avian sarcoma-leukosis viruses (ASV).

Structure, replication, and recombination of retrovirus genomes: some unifying hypotheses.

A rapid assay for retroviral reverse transcriptase activity released into the culture medium by infected cells was developed. With the assay, 4,000 clonally infected cell lines could be tested in a few hours. We have adapted the assay for use as a screen for the detection of spontaneous viral mutants. Mutants of Moloney murine leukemia virus have been isolated which (i) produce a thermolabile reverse transcriptase, (ii) are temperature sensitive for release of enzyme activity, or (iii) can only productively infect cells already producing gag-related polypeptides. The assay has also been useful for the isolation of nonproducer cells infected with various replication-defective transforming viruses.

William S. Mason

Papers

Genetic recombinants and heterozygotes derived from endogenous and exogenous avian RNA tumor viruses

Characterization of two conditional early mutants of Rous sarcoma virus

Determination of the defective function in two mutants of Rous sarcoma virus.

A replication defective mutant of Rous sarcoma virus which fails to make a functional reverse transcriptase.

Temperature-sensitive mutants of avian sarcoma viruses: genetic recombination between multiple or coordinate mutants and avian leukosis viruses.