Two internal sequence elements modulate transcription from the external human 7S K RNA gene promoter in vivo

Constructs of the external promoter of the human 7S K RNA gene in combination with different reporter elements and varying amounts of 7S K internal sequences were analyzed for efficient transcription by RNA polymerase III (pol III) in vitro and in vivo. In vitro, the 7S K promoter alone (-245 to -1) revealed full activity, compared to the entire wild-type gene. In vivo, however, the activity of the gene-external 7S K promoter, albeit clearly functional by itself, was positively modulated by internal sequence elements. Fusion constructs containing increasing amounts of transcribed 7S K sequences revealed that two elements were responsible for this activation. One element is associated with the initiator region (+1 to +8) of this class III gene. The second sequence comprises the 5' half of a cryptic A-box starting at +10 of the 7S K RNA sequence. In the context of a totally unrelated vector sequence, a GGC element alone was sufficient to functionally replace that cryptic A-box. Thus, it appears that in context of the 7S K RNA gene--and possibly the 7S L and U6 RNA genes as well--structurally divergent A-box-like elements function as internal modulators of these pol III promoters.