ABSTRACT
Specific transcription by RNA polymerase III requires recognition of the promoter-bound transcription factor IIIB (TFIIIB), of which the TATA-binding protein (TBP) is a subunit. The recruitment of TFIIIB to TATA-less genes is mediated by protein-protein interactions with transcription factor IIIC (TFIIIC) bound to the box A and box B elements. Here we examine interactions involved in the recruitment of TFIIIB to the TATA element-containing yeast U6 small nuclear RNA gene SNR6. TFIIIC is not required for the formation of TFIIIB-SNR6 gene complexes with purified components. The same three components of TFIIIB that are necessary for TFIIIC-dependent transcription of tRNA genes (recombinant TBP and Brf and the denaturing-gel-purified 90-kDa subunit) are required and sufficient for TATA box-directed U6 transcription. Despite its TFIIIC-independent, DNA sequence-dependent assembly, the TFIIIB-SNR6 complex shares important features with tDNA- and 5S rDNA-TFIIIB complexes, such as extent and location of footprint, stability, and resistance to heparin. These properties are clearly distinct from those of a TBP-SNR6 complex. In the SNR6 gene, box B, the primary binding site for TFIIIC, is suboptimally spaced relative to box A. At limiting TBP concentrations and on bare DNA, TFIIIC stimulates the formation of TFIIIB complexes with SNR6 but contributes poorly, at best, to the formation of properly placed complexes.
