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

Abstract

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 (I alpha) are transcriptionally inactive, hRRN3 defines a distinct subpopulation of Pol I complexes (I beta) 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 beta 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.

RNA polymerase I structure and transcription regulation

“…8b, c). Consistent with this, inactive human Pol I may be dimeric, whereas human Pol I complexes containing the Rrn3 homologue transcription initiation factor-IA (TIF-IA) are active 41 . Like Rrn3, human TIF-IA binds A43 (ref.…”

Multiple interactions between RNA polymerase I, TIF‐IA and TAF I subunits regulate preinitiation complex assembly at the ribosomal gene promoter

“…Moreover, TIF-IA has been shown to interact with TIF-IB/SL1 (Miller et al, 2001), which suggests that TIF-IA bridges both protein complexes. To assess the function of TIF-IA in initiation complex formation, Pol I , amino-acidstarved (lanes 6 and 7) and cycloheximide-treated (lanes 8 and 9) NIH 3T3 cells were assayed for their capability to restore the transcriptional activity of a nuclear extract from density-arrested FM3A cells (lane 1).…”

“…Besides being associated with Pol I, TIF-IA has been shown to interact with TIF-IB/SL1 (Miller et al, 2001). This suggests that, by interacting with both Pol I and TIF-IB/SL1, TIF-IA targets transcriptionally active Pol I molecules to the rDNA promoter.…”

Does Pol I talk to Pol II? Coordination of RNA polymerases in ribosome biogenesis: Figure 1.

“…Upon rapamycin treatment, Rrn3 dissociates from the Pol I complex, resulting in an arrest of transcription (Milkereit and Tschochner 1998;Miller et al 2001;Grummt 2003). In the CARA strain, however, the Rrn3-A43 fusion protein is not dissociable, and Pol I transcription was much less affected.…”