The role of TBP in rDNA transcription by RNA polymerase I in Saccharomyces cerevisiae: TBP is required for upstream activation factor-dependent recruitment of core factor.

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Abstract
Transcription of Saccharomyces cerevisiae rDNA by RNA polymerase I involves at least two transcription factors characterized previously: upstream activation factor (UAF) consisting of Rrn5p, Rrn9p, Rrn10p, and two more uncharacterized proteins; and core factor (CF) consisting of Rrn6p, Rrn7p, and Rrn11p. UAF interacts directly with an upstream element of the promoter and mediates its stimulatory function, and CF subsequently joins a stable preinitiation complex. The TATA-binding protein (TBP) has been known to be involved in transcription by all three nuclear RNA polymerases. We found that TBP interacts specifically with both UAF and CF, the interaction with UAF being stronger than that with CF. Using extracts from a TBP (I143N) mutant, it was shown that TBP is required for stimulation of transcription mediated by the upstream element, but not for basal transcription directed by a template without the upstream element. By template competition experiments, it was shown that TBP is required for UAF-dependent recruitment of CF to the rDNA promoter, explaining the TBP requirement for stimulatory activity of the upstream element. We also studied protein-protein interactions and found specific interactions of TBP with Rrn6p and with Rrn9p both in vitro and in the yeast two-hybrid system in vivo. Thus, these two interactions may be involved in the interactions of TBP with CF and UAF, respectively, contributing to the recruitment of CF to the rDNA promoter. Additionally, we observed an interaction between Rrn9p and Rrn7p both in vitro and in the two-hybrid system; thus, this interaction might also contribute to the recruitment of CF.