Trf4 targets ncRNAs from telomeric and rDNA spacer regions and functions in rDNA copy number control

Abstract

Trf4 is the poly(A) polymerase component of TRAMP4, which stimulates nuclear RNA degradation by the exosome. We report that in Saccharomyces cerevisiae strains lacking Trf4, cryptic transcripts are detected from regions of repressed chromatin at telomeres and the rDNA intergenic spacer region (IGS1‐R), and at CEN3. Degradation of the IGS1‐R transcript was reduced in strains lacking TRAMP components, the core exosome protein Mtr3 or the nuclear‐specific exosome component Rrp6. IGS1‐R has potential binding sites for the RNA‐binding proteins Nrd1/Nab3, and was stabilized by mutation of Nrd1. IGS1‐R passes through the replication fork barrier, a region required for rDNA copy number control. Strains lacking Trf4 showed sporadic changes in rDNA copy number, whereas loss of both Trf4 and either the histone deacetylase Sir2 or the topoisomerase Top1 caused dramatic loss of rDNA repeats. Chromatin immunoprecipitation analyses showed that Trf4 is co‐transcriptionally recruited to IGS1‐R, consistent with a direct role in rDNA stability. Co‐transcriptional RNA binding by Trf4 may link RNA and DNA metabolism and direct immediate IGS1‐R degradation by the exosome following transcription termination.