Nonsense Codons within the Rous Sarcoma Virus gag Gene Decrease the Stability of Unspliced Viral RNA

The intracellular accumulation of the unspliced RNA of Rous sarcoma virus was decreased when translation was prematurely terminated by the introduction of nonsense codons within its 5' proximal gene, the gag gene. In contrast, the levels of spliced viral RNAs were not affected in our transient expression assays in chicken cells. Experiments using the transcription inhibitor dactinomycin showed that mutant unspliced RNAs were degraded more rapidly than wild-type RNA. Furthermore, mutant RNAs could be partially stabilized by coexpression of wild-type gag proteins in trans; however, intact gag proteins were not required to maintain the stability of RNAs which did not contain premature termination codons. Thus, termination codons seemed to destabilize the RNA not because of their effect on gag protein function but instead because they disrupted the process of translating the gag region of the RNA. Analysis of double-mutant constructs containing both deletions and termination codons within the gag gene also suggested that the stability of the unspliced RNA was affected by a cis-acting interaction between the RNA and ribosomes.