Expression of a human 7S K RNA gene in vivo requires a novel pol III upstream element

The 5'-flanking sequences required for expression of a human 7S K RNA gene have been defined by mutant analysis. A -111 upstream deletion mutant showed full activity when analysed by in vitro transcription with HeLa cell extracts. In contrast, upon transfection into intact cells, this mutant only revealed a basal level activity of approximately 6% as compared to the wild-type promoter up to position -252. The deleted upstream sequence element acts as a transcriptional activator in vivo, in a strictly position and orientation-dependent manner. Two octamer-like binding motifs observed within this upstream sequence were both dispensable for proper function of this RNA polymerase III promoter in vivo. Instead, a detailed analysis of this region identified a CACCC-box element, together with its surrounding base pairs, as the essential upstream element required for expression of this 7S K RNA gene in vivo. Furthermore, this CACCC-box is centered within a footprint obtained with HeLa cell nuclear proteins.