RNA Polymerase I and Fob1 contributions to transcriptional silencing at the yeast rDNA locus.

RNA polymerase II (Pol II)-transcribed genes embedded within the yeast rDNA locus are repressed through a Sir2-dependent process called 'rDNA silencing'. Sir2 is recruited to the rDNA promoter through interactions with RNA polymerase I (Pol I), and to a pair of DNA replication fork block sites (Ter1 and Ter2) through interaction with Fob1. We utilized a reporter gene (mURA3) integrated adjacent to the leftmost rDNA gene to investigate localized Pol I and Fob1 functions in silencing.

Pnc1p-mediated nicotinamide clearance modifies the epigenetic properties of rDNA silencing in Saccharomyces cerevisiae.

The histone deacetylase activity of Sir2p is dependent on NAD(+) and inhibited by nicotinamide (NAM). As a result, Sir2p-regulated processes in Saccharomyces cerevisiae such as silencing and replicative aging are susceptible to alterations in cellular NAD(+) and NAM levels. We have determined that high concentrations of NAM in the growth medium elevate the intracellular NAD(+) concentration through a mechanism that is partially dependent on NPT1, an important gene in the Preiss-Handler NAD(+) salvage pathway.

Diversity in the Sir2 family of protein deacetylases.

The silent information regulator (Sir2) family of protein deacetylases (Sirtuins) are nicotinamide adenine dinucleotide (NAD)(+)-dependent enzymes that hydrolyze one molecule of NAD(+) for every lysine residue that is deacetylated. The Sirtuins are phylogenetically conserved in eukaryotes, prokaryotes, and Archeal species. Prokaryotic and Archeal species usually have one or two Sirtuin homologs, whereas eukaryotes typically have multiple versions.

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.