Both RNA polymerase III and RNA polymerase II accurately initiate transcription from a human U6 promoter in vitro.

The promoter of vertebrate U6 small nuclear RNA genes consists of a TATA box and a snRNA proximal sequence element (PSE), and the combination of these two elements directs RNA polymerase III transcription. We detected RNA polymerase II transcription as well as pol III transcription from the human U6 promoter in a HeLa nuclear extract. The pol II-specific transcription was independent of the PSE and dependent on the presence of the TATA box. Both pol III- and pol II-specific transcription were stimulated by addition of recombinant TATA-binding protein (TBP). We conclude that both pol III and pol II preinitiation complexes can assemble on the U6 promoter in vitro and could compete during the bona fide process in the cell.