hRRN3 is essential in the SL1‐mediated recruitment of RNA Polymerase I to rRNA gene promoters

A crucial step in transcription is the recruitment of RNA polymerase to promoters. In the transcription of human rRNA genes by RNA Polymerase I (Pol I), transcription factor SL1 has a role as the essential core promoter binding factor. Little is known about the mechanism by which Pol I is recruited. We provide evidence for an essential role for hRRN3, the human homologue of a yeast Pol I transcription factor, in this process. We find that whereas the bulk of human Pol I complexes (Iα) are transcriptionally inactive, hRRN3 defines a distinct subpopulation of Pol I complexes (Iβ) that supports specific initiation of transcription. Human RRN3 interacts directly with TAF I 110 and TAF I 63 of promoter‐selectivity factor SL1. Blocking this connection prevents recruitment of Pol I β to the rDNA promoter. Furthermore, hRRN3 can be found in transcriptionally autonomous Pol I holoenzyme complexes. We conclude that hRRN3 functions to recruit initiation‐competent Pol I to rRNA gene promoters. The essential role for hRRN3 in linking Pol I to SL1 suggests a mechanism for growth control of Pol I transcription.