Abstract
The 5'-proximal open reading frame on Rous sarcoma virus RNA encodes a seven-amino-acid peptide and is conserved in all avian sarcoma-leukosis retroviruses. Ribosome-binding site analysis in intact chick cells showed that the 5'-proximal AUG codon is a strong site for initiation of translation in vivo. Removal of the 5'-proximal AUG codon by site-specific mutagenesis resulted in a virus with a reduced ability either to replicate or to transform a population of chicken embryo fibroblasts. These results establish a procedure for determining sites of ribosome binding and initiation of translation on mRNAs in intact eucaryotic cells and strongly suggest that the 5'-proximal open reading frame (or its AUG codon) on Rous sarcoma virus RNA has an important role in regulating viral activity.