EFFECTS OF THE OPEN READING FRAMES IN THE ROUS-SARCOMA VIRUS LEADER RNA ON TRANSLATION

Three short open reading frames (ORFs) reside in the 5' leader of Rous sarcoma virus (RSV) and are conserved in all avian sarcoma-leukosis r etroviruses. Both extensions of the lengths of the ORFs and alteration s in their initiation codons affect viral replication and gene express ion. To determine whether the effects on viral replication were due to translational regulation mediated by the ORFs, we examined translatio n following mutation of the initiation and termination codons of each of the three ORFs. We found that the ORFs marginally enhanced downstre am gene expression. Moreover, repression of downstream gene translatio n was proportional to the lengths of the elongated ORFs and depended o n the initiation contexts of the AUG codons. Although the ORFs play a major role in viral activities, their effects on translation were rela tively minor. Rather, the ORFs may affect the fate of unspliced avian retroviral RNA in chronically infected cells by participating in the s orting of viral RNA for either translation or encapsidation into virio ns.