The genesis of rous sarcoma virus messenger RNAs

Cells infected with Rous sarcoma virus (RSV) produce three classes of virus-specific mRNAs-one that has the size of the viral genome and two that are smaller than the genome. We have explored the mechanisms by which these viral mRNAs are produced. The site for initiation of viral RNA synthesis was located by using irradiation with ultraviolet light to inactivate transcription in infected cells. The metabolism of viral RNA was elucidated by the use of glucosamine to effect satisfactory pulse-chase experiments. We conclude that transcription from the RSV provirus initiates at or near the nucleotide that encodes the 5′ terminus of the viral genome. The primary transcript is subsequently processed to generate the subgenomic mRNAs, which appear independently of each other between 15 and 45 min after the synthesis of their precursor. Viral RNA has several distinctive fates: ca. 35% remains in the cell in the form of the genomic length mRNA; 10-20% takes the form fo subgenomic mRNAs; and the remaining 50% disappears from the cell, presumably because of export as viral genome (but perhaps also as a consequence of degradation within the cell). Our data provide a functional definition for the initiation of viral RNA synthesis and conform to the results of recent structural studies that have identified a possible "promoter site" near the ends of the RSV provirus.