Synthesis of murine leukemia viral DNA in vitro: evidence for plus-strand DNA synthesis at both ends of the genome.

We studied the synthesis of B-tropic murine leukemia viral DNA in vitro by detergent-disrupted virions. The reaction products (detected by the Southern transfer technique) included full-length, infectious, double-stranded DNA and several subgenomic fragments. Restriction endonuclease analysis and hybridization and specific probes revealed two classes of subgenomic fragments: some were derived from the right end of the genome, and some were derived from the left end. Most of the fragments harbored one long terminal repeat copy at their ends, suggesting that they were initiated correctly. S1 nuclease and restriction endonuclease treatments of these fragments indicated that a single-stranded gap was present near the first initiation site of plus strong-stop DNA. The treatments also suggested the presence of a second initiation site flanked by a single-stranded gap 0.9 kilobase pairs from the right end of the genome. Our data clearly show that plus-strand DNA is synthesized at both ends of the genome, by using plus strong stop as the first initiation site and additional initiation sites.