Basal promoter and enhancer element of yeast U6 snRNA gene.

The yeast U6 snRNA gene, SNR6, transcribed by RNA polymerase III or C, is shown to have a mixed promoter with upstream, intragenic and downstream elements. The distant downstream B block behaves as a typical enhancer element. Required in vivo, and for transcription of chromatin templates in vitro, it was also active in reversed orientation. As shown by footprinting and electron microscopy, the factor TFIIIC, or tau, bound the B block in an oriented manner and was able to induce DNA looping. The factor TFIIIC appeared to act via a weak A block located at position +21. This A block-related motif was essential in vivo and with chromatin templates. When changed into a consensus A block it favored DNA looping by TFIIIC firmly anchored on the B block, and activated a B block lacking gene in vivo and in vitro. The role of the TATA box at -30 was most apparent using a purified transcription system. With the A block, it appeared to contribute to start site selection. The results suggest a model where three weak promoter elements collaborate to assemble the transcription complex by DNA looping and synergistic protein-DNA interactions.