The terminal redundancy of the retrovirus genome facilitates chain elongation by reverse transcriptase

Transcription of DNA from the RNA genome of retroviruses by reverse transcriptase involves an unusual translocation of the growing chain from the 5' end to the 3' end of the RNA template. In order to elucidate the mechanism by which this translocation occurs, we have used chain termination to analyze nascent viral DNA synthesized in vitro by avian sarcoma virus, and we have determined the nucleotide sequence of appropriate regions of viral DNA isolated from infected cells and cloned into prokaryotic vectors. Our results provide direct experimental evidence for a previously proposed model in which a short terminal redundancy in viral RNA, and a DNA copy of the redundant sequence, are used to allow the growing DNA chain to move from the 5' to the 3' end of the template. Transcription of avian sarcoma virus RNA with purified reverse transcriptase also generates an anomalous product, a hairpin DNA that arises when the initial DNA transcript folds back on itself to continue synthesis. The foldback is mediated by an inverted repeat of 5 nucleotides in the sequence of nascent DNA. Anomalous hairpin DNA is not produced by detergent-activated virions. Thus, constituents of the virons or the configuration of encapsidated viral RNA must faciliatate correct transcription

Transduction of a cellular oncogene: The genesis of Rous sarcoma virus

The oncogene of Rous sarcoma virus (v-src) arose by transduction of a cellular gene (c-src). In an effort to explore the mechanism of transduction, we have identified the splice acceptor site used in the genesis of mRNA for v-src, shown that an equivalent site is used in the splicing of mRNA for c-src, and determined the nucleotide sequence from the boundaries of homology between v-src and c-src. Our data indicate that (i) only a portion of c-src is represented within v-src, (ii) the leftward recombination between the genome of the transducing virus and c-src occurred in an intron of the cellular gene, (iii) v-src is in part a spliced version of the corresponding portion of c-src, and (iv) nucleotide sequences represented once in the genome of the transducing virus become duplicated to flank v-src. These findings indicate that the first step in transduction is probably recombination between DNA forms of the transducing viral genome and c-src and otherwise support the prevailing model for transduction by retroviruses. The carboxyl termini of the proteins encoded by v-src and c-src differ appreciably. An unidentified domain of 127 or 128 nucleotides is located at different positions in the genomes of two strains of RSV and gives evidence of being a foreign element that entered the viral genomes by genetic transposition independent of the transduction of src