Role of TATA binding protein (TBP) in yeast ribosomal DNA transcription byRNA polymerase I: Defects in the dual functions of transcription factor UAF cannot be suppressed by TBP

Initiation of ribosomal DNA (rDNA) transcription by RNA polymerase I (Pol I ) in the yeast Saccharomyces cerevisiae involves upstream activation factor (UAF), core factor, the TATA binding protein (TBP), and Rrn3p in addition to Pol I. We found previously that yeast strains carrying deletions in the UAF component RRN9 switch completely to the use of Pol II for rRNA transcri ption, with no residual Pol I transcription, These polymerase-snitched stra ins initially grow very slowly, but subsequent expansion in the number of r DNA repeats on chromosome XII leads to better growth. Recently, it was repo rted that TBP overexpression could bypass the requirement of UAF for Pol I transcription in vivo, producing nearly wild-type levels of growth in UAF m utant strains (P. Aprikian, B. Moorefield, and R.H. Reeder, Mel. Cell. Biol . 20:5269-5275, 2000). Here, we demonstrate that deletions in the UAF compo nent RRN5, RRN9, or RRN10 lead to Pol II transcription of rDNA. TBP overexp ression does not suppress UAF mutation, and these strains continue to nse P ol II for rRNA transcription. We do not find evidence for even low levels o f Pol I transcription in UAF mutant strains carrying overexpressed TBP. In diploid strains lacking both copies of the UAF component RRN9, Pol II trans cription of rDNA is more strongly repressed than in haploid strains but TBP overexpression still fails to activate Pol I. These results emphasize that UAF plays an essential role in activation of Pol I transcription and silen cing of Pol II transcription of rDNA and that TBP functions to recruit the Pol I machinery in a manner completely dependent on UAF.