cis-Acting RNA packaging locus in the 115-nucleotide direct repeat of Rous sarcoma virus.

The v-src gene of Schmidt-Ruppin strain A of Rous sarcoma virus is flanked by a 115-nucleotide direct repeat. Mutants that lack either the upstream or downstream copy replicate normally. However, mutants that lack both copies do not replicate. Cloned viral DNA lacking both copies of the 115-nucleotide sequence is capable of directing the transcription of viral RNA posttransfection. This viral RNA is polyadenylated, spliced, exported from the nucleus, and translated into protein normally. However, virions isolated from the culture medium 48 h posttransfection lack viral RNA. When mutant DNA is contransfected with wild-type DNA, the virions produced 48 h later contain wild-type RNA but not mutant RNA, even though both RNAs are present in the cytoplasm. We propose that the 115-nucleotide element of Rous sarcoma-avian leukosis virus encodes a cis-acting sequence that is necessary for the proper incorporation of viral RNA into virions.