Infection of terminally differentiated myotubes with Rous sarcoma virus: reduced synthesis of env and v-src proteins.

We studied Rous sarcoma virus (RSV) protein synthesis in RSV-infected, terminally differentiated chicken myotubes ('late-infected' myotubes), in which no viral DNA integration takes place but all three viral mRNAs (38S, 28S and 21S) are transcribed normally. With the use of specific anti-RSV protein antisera, we found that only the viral gag and pol proteins were synthesized at levels similar to those synthesized in RSV-transformed fibroblasts; the synthesis of env and v-src proteins was significantly reduced in these infected myotubes. We concluded that the viral RNA transcribed from the unintegrated RSV DNA was functional but that genes at the 3' end of the RSV genome were translated at a lower level. By contrast, when mononucleated replicating chicken myoblasts were infected with a mutant (tsNY68) carrying a temperature-sensitive v-src gene and maintained at the non-permissive temperature for this gene, they developed into myotubes with viral DNA integrated in their chromosomal DNA. These 'early-infected' myotubes expressed all four viral genes (gag, pol, env and v-src) at a level similar to that in infected fibroblasts. This result ruled out the possible presence of specific factor(s) in myotubes that preferentially inhibit the 3' genes of RSV, and suggested other translational control(s) of viral gene expression in late-infected myotubes.