The repressor sequence upstream of c-mos acts neither as polyadenylation site nor as transcription termination region.

Recently we reported that the c-mos(rat) coding region is preceded by sequences (RIS) which repress accumulation of c-mos RNA in the cytoplasm. To investigate the effect of RIS on RNA transcription or processing a retroviral promoter was inserted at different positions relative to RIS. Cotransfection was used to establish cell lines with high copy number of the plasmids and to avoid any selection for c-mos expression or RIS function. Analysis of RNA in the cell lines indicated that RIS does not provide a poly(A) site and allowed characterization of the c-mos(rat) poly(A) site. Surprisingly, RIS contains sequences homologous to elements involved in eucaryotic RNA cleavage/polyadenylation. To determine an effect of RIS on transcription, RNA was elongated in vitro in nuclei isolated from the cell lines and used to analyze the number of RNA polymerase II molecules transcribing different regions of the plasmid. The analysis showed that RIS does not act as transcription termination region.