Establishment of closed 35S ribosomal RNA gene chromatin in stationary Saccharomyces cerevisiae cells.

As a first step in eukaryotic ribosome biogenesis RNA polymerase (Pol) I synthesizes a large ribosomal RNA (rRNA) precursor from multicopy rRNA gene loci. This process is essential for cellular growth and regulated in response to the cell's physiological state. rRNA gene transcription is downregulated upon growth to stationary phase in the yeast Saccharomyces cerevisiae. This reduction correlates with characteristic changes in rRNA gene chromatin structure from a transcriptionally active 'open' state to a non-transcribed 'closed' state. The conserved lysine deacetylase Rpd3 was shown to be required for this chromatin transition. We found that Rpd3 is needed for tight repression of Pol I transcription upon growth to stationary phase as a prerequisite for the establishment of the closed chromatin state. We provide evidence that Rpd3 regulates Pol I transcription by adjusting cellular levels of the Pol I preinitiation complex component core factor (CF). Importantly, our study identifies CF as the complex limiting the number of open rRNA genes in exponentially growing and stationary cells.