The recruitment of RNA polymerase I on rDNA is mediated by the interactionof the A43 subunit with Rrn3

RNA polymerase I (Pol I) is dedicated to transcription of the large ribosom al DNA (rDNA). The mechanism of Pot I recruitment onto rDNA promoters is po orly understood. Here we present evidence that subunit A43 of Pol I interac ts with transcription factor Rrn3: conditional mutations in A43 were found to disrupt the transcriptionally competent Pol I-Rrn3 complex, the two prot eins formed a stable complex when coexpressed in Escherichia coli, overexpr ession of Rrn3 suppressed the mutant phenotype, and A43 and Rrn3 mutants sh owed synthetic lethality. Consistently, immunoelectron microscopy data show ed that A43 and Rrn3 co-localize within the Pol I-Rrn3 complex. Rrn3 has se veral protein partners: a two-hybrid screen identified the C-terminus of su bunit Rrn6 of the core factor as a Rrn3 contact, an interaction supported i n vitro by affinity chromatography. Our results suggest that Rrn3 plays a c entral role in Pol I recruitment to rDNA promoters by bridging the enzyme t o the core factor. The existence of mammalian orthologues of A43 and Rrn3 s uggests evolutionary conservation of the molecular mechanisms underlying rD NA transcription in eukaryotes.