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Gene transcription; rRNA production; ribosome biogenesis; promoter specificity; gene class-specific RNA synthesis
Abstract:
Cell growth is regulated during RNA polymerase (Pol) I transcription initiation by the conserved factor Rrn3/TIFIA
in yeast/humans. Here we provide a structure–function analysis of Rrn3 based on a combination of structural
biology with in vivo and in vitro functional assays. The Rrn3 crystal structure reveals a unique HEAT repeat fold
and a surface serine patch. Phosphorylation of this patch represses human Pol I transcription, and a phosphomimetic
patch mutation prevents Rrn3 binding to Pol I in vitro and reduces cell growth and Pol I gene occupancy
in vivo. Cross-linking indicates that Rrn3 binds Pol I between its subcomplexes, AC40/19 and A14/43, which faces
the serine patch. The corresponding region of Pol II binds the Mediator head that cooperates with transcription
factor (TF) IIB. Consistent with this, the Rrn3-binding factor Rrn7 is predicted to be a TFIIB homolog. This reveals
the molecular basis of Rrn3-regulated Pol I initiation and cell growth, and indicates a general architecture of
eukaryotic transcription initiation complexes.
