Proposed gag-encoded transcriptional activator is not necessary for Rous sarcoma virus replication or transformation

It has been reported that gene expression directed by the long terminal repeat of Rous sarcoma virus (RSV) is trans activated by a protein encoded in an alternate reading frame within the RSV gag gene (S. Broome and W. Gilbert, Cell 40:537-546, 1985). We have made specific mutations to test the role of the putative transcriptional activator in RSV replication. Termination codons were created within the alternate reading frame coding for the trans activator, and the mutations were introduced into an infectious RSV plasmid. We were unable to demonstrate specific trans activation of the RSV long terminal repeat by either wild-type or mutant RSV plasmids in transient cotransfection assays. Experiments using mutant or wild-type RSV-infected chick embryo fibroblasts indicated that the proposed RSV transcriptional activator was not required for viral replication or transformation and did not increase steady-state levels of viral RNA.