Sequences upstream from the mouse c-mos oncogene may function as a transcription termination signal

A region upstream from the mouse c-mos proto-oncogene, termed upstream mouse sequence (UMS), prevents expression of mos transforming activity. Previous studies suggested that the UMS prevented transcription readthrough. In this study, we constructed a recombinant DNA clone, pHTS3MS, with the UMS inserted downstream from both the mos gene and a truncated long terminal repeat containing only the U3 enhancer region. In this position UMS did not inhibit mos transforming activity. We examined cells transformed by pHTS3MS for RNA expression. S1 nuclease analysis showed that the UMS provides two polyadenylation signals to mos-containing RNA and nuclear run-on transcription showed that the primary transcripts terminate in UMS. In addition, using portions of the UMS, we found that a 360-bp fragment containing the UMS polyadenylation signals and sites inserted between the herpes simplex virus type 1 (HSV-1) thymidine kinase gene (tk) and its promoter inhibits tk transforming activity by 99% and prevents detectable expression of this construct in transient expression assays. Thus, the UMS must contain signals for polyadenylation and appears to function as a transcription terminator.