Co-transcriptional RNA cleavage provides a failsafe termination mechanism for yeast RNA polymerase i

Abstract

Ribosomal RNA, transcribed by RNA polymerase (Pol) I, accounts for most cellular RNA. Since Pol I transcribes rDNA repeats with high processivity and polymerase density, transcription termination is a critical process. Early in vitro studies proposed polymerase pausing by Reb1 and transcript release at the T-rich element T1 determined transcription termination. However recent in vivo studies revealed a 'torpedo' mechanism for Pol I termination: co-transcriptional RNA cleavage by Rnt1 provides an entry site for the 5′-3′ exonuclease Rat1 that degrades Pol I-associated transcripts destabilizing the transcription complex. Significantly Rnt1 inactivation in vivo reveals a second co-transcriptional RNA cleavage event at T1 which provides Pol I with an alternative termination pathway. An intact Reb1-binding site is also required for Rnt1-independent termination. Consequently our results reconcile the original Reb1-mediated termination pathway as part of a failsafe mechanism for this essential transcription process. The Author(s) 2010. Published by Oxford University Press.2010This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http:// creativecommons.org/licenses/by-nc/2.5), which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. © The Author(s) 2010.