INHIBITION OF RNA SPLICING AT THE ROUS-SARCOMA VIRUS SRC-3' SPLICE-SITE IS MEDIATED BY AN INTERACTION BETWEEN A NEGATIVE CIS-ELEMENT AND A CHICKEN-EMBRYO FIBROBLAST NUCLEAR FACTOR

In permissive Rous sarcoma virus-infected chicken embryo fibroblasts ( CEF), approximately equimolar amounts of env and src mRNAs are present . In nonpermissive mammalian cells, the src mRNA level is elevated and env mRNA level is reduced. A cis element in the region between the en v gene and the src 3' splice site, which we have termed the suppressor of src splicing (SSS), acts specifically in CEF but not in human cell s to reduce src mRNA levels. The splicing inhibition in CEF is not cau sed by a base-paired structure which is predicted to form between the SSS and the are 3' splice site. To further investigate the mechanism o f the inhibition, we have used human HeLa cell nuclear extracts to com pare in vitro the rates of splicing of RNA substrates containing the R ous sarcoma virus major 5' splice site and either the env or src 3' sp lice sites. We show that the src 3' splice site is used approximately fivefold more efficiently than the env 3' splice site. The efficiency of in vitro splicing at the src 3' splice site is specifically reduced by addition of CEF nuclear extract. The inhibition is dependent on th e presence of the SSS element and can be abrogated by addition of comp etitor RNA. We propose that the SSS region represents a binding site f or a negative-acting CEF splicing factor(s).