Cartoon diagram of factors required for transcription initiation by Pol I. (A) Yeast factors are illustrated with subunit names indicated. Subunits of individual factors are colored the same. Separable elements in the promoter DNA sequence have been characterized and are indicated. HmoI is indicated downstream of the core promoter, but may also influence transcription initiation. (B) Mammalian factors are colored to reflect their functional analogy to the yeast factors. UBF is shown at the promoter, but it also associates with the coding region of the rDNA. Notably, TBP is required in yeast and mammals, but original characterization of SL1 included TBP as a subunit, thus it is colored as such. TAF41 and TAF12 were recently shown to bind SL1 (Denissov et al., 2007; Gorski et al., 2007); however their direct requirement for activity of SL1 in vitro has not yet been demonstrated. The transcription termination factor, TTF-I, also affects transcription initiation by Pol I and is shown bound to one of several sites upstream of the Pol I promoter.

Diagram of the ribosomal DNA repeat. The promoter for Pol I is indicated by a bent arrow. Mature rRNAs (indicated in dark blue) are processed out of the precursor RNA (35S pre-rRNA in yeast; 45S, in human). RNAs that are not present in the ribosome (external transcribed sequence, “ETS” and internally transcribed sequences “ITS1” and “ITS2” are in light blue). In yeast, the gene that encodes 5S rRNA is interspersed between 35S rRNA repeats, whereas in other eukaryotes the 5S gene is located in a separate locus.