Iwr1 protein is important for preinitiation complex formation by all three nuclear RNA polymerases in Saccharomyces cerevisiae

Background: Iwr1, a protein conserved throughout eukaryotes, was originally identified by its physical interaction with RNA polymerase (Pol) II. Principal Findings: Here, we identify Iwr1 in a genetic screen designed to uncover proteins involved in Pol III transcription in S. cerevisiae. Iwr1 is important for Pol III transcription, because an iwr1 mutant strain shows reduced association of TBP and Pol III at Pol III promoters, a decreased rate of Pol III transcription, and lower steady-state levels of Pol III transcripts. Interestingly, an iwr1 mutant strain also displays reduced association of TBP to Pol I-transcribed genes and of both TBP and Pol II to Pol II-transcribed promoters. Despite this, rRNA and mRNA levels are virtually unaffected, suggesting a post-transcriptional mechanism compensating for the occupancy defect. Conclusions: Thus, Iwr1 plays an important role in preinitiation complex formation by all three nuclear RNA polymerases.