Genetic analysis of RNA polymerase I unveils new role of the Rpa12 subunit during transcription

Most transcriptional activity of exponentially growing cells is carried out by RNA Polymerase I (Pol I), which produces a large rRNA precursor. The Pol I transcription cycle is achieved through complex structural rearrangements of the enzyme, revealed by recent structural studies. In the yeast S. cerevisiae the Pol 1 subunit Rpa49, particularly its C-terminal tandem winged helix domain (Rpa49Ct), is required supports both initiation and elongation of the transcription cycle. Here, we characterized novel extragenic suppressors of the growth defect caused by the absence of Rpa49. We identified suppressor mutations on the two largest subunits of Pol I, Rpa190 and Rpa135, as well as Rpa12. Suppressor mutants RPA135-F301S and RPA12-S6L restored normal rRNA synthesis and increased Pol I density on rDNA genes in the absence of Rpa49Ct. Most mutated residues cluster at an interface formed by the jaw in Rpa190, the lobe in Rpa135, and subunit Rpa12 when mapped on the structure of Pol I. Our genetic data in S. cerevisiae suggest a new role for Rpa12 at the jaw/lobe interface during transcription cycle.