Transcription of a yeast U6 snRNA gene requires a polymerase III promoter element in a novel position.

Vertebrate genes coding for U6 small nuclear RNA are transcribed by RNA polymerase III (pol III), using only upstream promoter elements rather than the A and B block internal control regions typical of most pol III transcription units. We show that expression of the U6 gene from the yeast Saccharomyces cerevisiae has two unexpected features: it requires a B block promoter element, and this element is located in a novel position, 120 bp downstream of the coding region. In tRNA genes, the B block is the primary binding site for transcription factor (TF) IIIC, whose function is to promote the subsequent binding of TFIIIB. Both factors are thus implicated in yeast U6 gene transcription. We present a model of the U6 transcription complex based on the structure of yeast and vertebrate U6 promoters.