Saccharomyces cerevisiae RNA Polymerase I Terminates Transcription at the Reb1 Terminator In Vivo

We have mapped transcription termination sites for RNA polymerase I in the yeast Saccharomyces cerevisiae. S1 nuclease mapping shows that the primary terminator is the Reb1p terminator located at +93 downstream of the 3′ end of 25S rRNA. Reverse transcription coupled with quantitative PCR shows that approximately 90% of all transcripts terminate at this site. Transcripts which read through the +93 site quantitatively terminate at a fail-safe terminator located further downstream at +250. Inactivation of Rnt1p (an RNase III involved in processing the 3′ end of 25S rRNA) greatly stabilizes transcripts extending to both sites and increases readthrough at the +93 site. In vivo assay of mutants of the Reb1p terminator shows that this site operates in vivo by the same mechanism as has previously been delineated through in vitro studies.