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Efficient termination of transcription by RNA polymerase I requires the 5' exonuclease Rat1 in yeast

¹ Wellcome Trust Centre for Cell Biology, University of Edinburgh, Edinburgh EH9 3JR, United Kingdom; ² Institute of Genetics and Biotechnology, Faculty of Biology, University of Warsaw, 02-106 Warsaw, Poland

During transcription termination by RNA polymerase II on protein-coding genes, the nuclear 5' exonuclease Rat1/Xrn2 degrades the nascent transcript downstream from the polyadenylation site and "torpedoes" the polymerase. We report that the activity of Rat1 is also required for efficient termination by RNA polymerase I (Pol I) on the rDNA. In strains lacking catalytically active Rat1 or its cofactor Rai1, Pol I reads through the major, "Reb1-dependent" terminator (T1) but stops downstream at the "fail-safe" terminator (T2) and replication fork barrier (RFB). The absence of both Rat1 and the RFB-binding protein Fob1 increased Pol I read-through of T2 and the RFB. We propose that cotranscriptional cleavage of the pre-rRNA by the endonuclease Rnt1 generates a loading site for the Rat1/Rai1 complex, which then degrades the nascent transcript. When Rat1 catches Pol I, which is predicted to be paused at T1, transcription is terminated.

[Keywords: Ribosome synthesis; rRNA synthesis; RNA polymerase I; exonuclease; transcription termination]]

Received November 14, 2007; revised version accepted February 29, 2008.

E-MAIL d.tollervey{at}ed.ac.uk; FAX 44-131-650-7040.

Supplemental material is available at http://www.genesdev.org.

Article is online at http://www.genesdev.org/cgi/doi/10.1101/gad.463708.

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