The 28 S genomic RNA of avian sarcoma virus PRCII codes for the transformation-specific polyprotein P105

We have characterized the genomic RNA of the defective avian sarcoma virus PRCII. Replicating virus which consists of transforming PRCII and a nontransforming but replication-competent helper, PRCII-AV, contains two RNA species. One is identical in size to the 35 S genome of avian leukosis helper viruses. The second component migrates slightly faster than 28 S ribosomal RNA in polyacrylamide gel electrophoresis but co-sediments with this RNA in sucrose gradients. In vitro translation across a gradient of velocity-sedimented poly(A)-containing PRCII virion RNA yielded three major proteins. The virion protein precursors Pr76gag and Pr180gag-pol were translated from 35 S RNA, while the transformation-specific polyprotein P105 was translated from 28 S RNA. P105 may be the only protein coded for by the PRCII genome, although this product would not exhaust the coding capacity of 28 S RNA. Whether translated in vitro or immunoprecipitated from transformed cells, P105 was essentially identical as demonstrated by comparative peptide maps