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. Author manuscript; available in PMC: 2013 Dec 11.
Published in final edited form as: Trends Biochem Sci. 2005 Feb;30(2):10.1016/j.tibs.2004.12.008. doi: 10.1016/j.tibs.2004.12.008

Figure 3. The RNA polymerase I (pol I) transcription cycle: pre-initiation complex formation (PIC), transcription initiation, promoter escape and clearance, elongation, termination and reinitiation.

Figure 3

(1) De novo PIC formation involves the selective binding of selectivity factor 1 (SL1) to the rDNA promoter, the incorporation of activator upstream binding protein (UBF) and (2) the recruitment of Pol Iβ by SL1. (3) Pol I initiates transcription upon promoter opening and, following promoter escape (3), pol I is converted into a processive enzyme (pol Iε), which elongates the nascent rRNA (4). (5) Transcription by pol I terminates at the 3′ end of the gene at specific sequences bound by termination factor TTF-I and transcript-release factor PTRF, with the concomitant release of pol I and the nascent rRNA. (6) SL1 and UBF remain promoter-bound following promoter clearance by pol I, and form a reinitiation scaffold onto which a pol Iβ complex, perhaps generated from recycled pol I and hRRN3, is recruited, and the resultant productive PIC can initiate another cycle of transcription.