RNA Polymerase I Propagates Unidirectional Spreading of rDNA Silent Chromatin

The ribosomal DNA (rDNA) tandem array in Saccharomyces cerevisiae induces transcriptional silencing of RNA polymerase II-transcribed genes. This SIR2-dependent form of repression (rDNA silencing) also functions to limit rDNA recombination and is involved in life span control. In this report, we demonstrate that rDNA silencing spreads into the centromere-proximal unique sequence located downstream of RNA polymerase I (Pol I) transcription, but fails to enter the upstream telomere-proximal sequences. The spreading of silencing correlates with SIR2–dependent histone H3 and H4 deacetylation and can be extended by SIR2 overexpression. Surprisingly, rDNA silencing required transcription by RNA polymerase I and the direction of spreading was controlled by the direction of Pol I transcription.