Evidence for circularization of the avian oncornavirus RNA genome during proviral DNA synthesis from studies of reverse transcription in vitro.

The RNA-directed DNA polymerase (deoxynucleosidetriphosphate:DNA deoxynucleotidyltransferase EC 2.7.7.7) of avian oncornavirus requires a tryptophan tRNA (tRNATrp) primer molecule located close to the 5' end of the viral RNA genome for the initiation of DNA synthesis in vitro. In this communication we demonstrate that the DNA product, transcribed from avian myeloblastosis virus (AMV) 35S RNA containing only tRNATrp as primer, is located also at the 5' end of the RNA genome. More importantly, we demonstrate that these 5' terminal DNA transcripts contain nucleotide sequences complementary to the 3' end of the genome. We have interpreted these results to mean that the genome. We have interpreted these results to mean that the 3' and 5' termini of the AMV 35S RNA genome become juxtaposed with each other either before or immediately after DNA synthesis has begun. These results are discussed in regard to the mechanism for synthesis of the circular forms of oncornavirus proviral DNA.