Academic journal article Genetics

Conserved Locus-Specific Silencing Functions of Schizosaccharomyces Pombe Sir2^sup +^

Academic journal article Genetics

Conserved Locus-Specific Silencing Functions of Schizosaccharomyces Pombe Sir2^sup +^

Article excerpt

ABSTRACT

In Schizosaccharomyces pombe, three genes, sir2^sup +^, hst2^sup +^, and hst4^sup +^, encode members of the Sir2 family of conserved NAD^sup +^-dependent protein deacetylases. The S. pombe sir2^sup +^ gene encodes a nuclear protein that is not essential for viability or for resistance to treatment with UV or a microtubule-destabilizing agent. However, sir2^sup +^ is essential for full transcriptional silencing of centromeres, telomeres, and the cryptic mating-type loci. Chromatin immunoprecipitation results suggest that the Sir2 protein acts directly at these chromosomal regions. Enrichment of Sir2p at silenced regions does not require the HP1 homolog Swi6p; instead, Swi6-GFP localization to telomeres depends in part on Sir2p. The phenotype of sir2 swi6 double mutants supports a model whereby Sir2p functions prior to Swi6p at telomeres and the silent mating-type loci. However, Sir2p does not appear to be essential for the localization of Swi6p to centromeric foci. Cross-complementation experiments showed that the Saccharomyces cerevisiae SIR2 gene can function in place of S. pombe sir2^sup +^, suggesting overlapping deacetylation substrates in both species. These results also suggest that, despite differences in most of the other molecules required, the two distantly related yeast species share a mechanism for targeting Sir2p homologs to silent chromatin.

SACCHAROMYCES cerevisiae Sir2p is a member of a ubiquitous protein family characterized by a conserved NAD+-dependent protein deacetylation domain (reviewed in BUCK et al. 2004). The family members are regulatory proteins that control a diverse set of functions from transcriptional silencing (reviewed in RUSCHE et al. 2003) to aspects of skeletal muscle differentiation (FuLCO et al. 2003). Consistent with these wide-ranging cellular roles, substrates of Sir2p family members include tubulin (NORTH et al 2003), p53 (Luo et al 2001; VAZIRI et al 2001; LANGLEY et al 2002), the archael chromatin protein Alba (BELL et al. 2002), an acyl-coenzyme A synthetase (STARAI et al. 2002), histones (reviewed in GOTTSCHLING 2000; SHORE 2000), a forkhead transcription factor (BRUNET et al. 2004; MOTTA et al. 2004), and the pol I subunit TAF(I)68 (MUTH et al 2001). The NAD+-dependent activity of Sir2p homologs may be critical to their roles as sensors of cellular metabolic states (GuARENTE 2000; SMITH et al 2000; SANDMEIER et al 2002; BITTERMAN et al 2003; DENU 2003). Although there has been a dramatic increase in the study of this family in recent years, no comprehensive understanding yet exists for the range of functions that the SIR2 gene family members perform in any one organism and the ways in which these functions vary from one species to another.

In S. cerevisiae, there are five members of the SIR2 gene family, SIR2 itself and the four HST (homolog of SIR two) genes. The S. cerevisiae SIR2 gene was first identified on the basis of its contribution to transcriptional silencing of the unexpressed copies of matingtype genes used as templates in mating-type switching (RLAR et al. 1979; RINE and HERSKOWITZ 1987). Further studies uncovered a role for Sir2p in silencing transcription of reporter genes inserted ectopically at telomeres (ApARICIO et al. 1991). Models for silencing at telomeres and the silent mating-type loci in S. cerevisiae propose that DNA-binding proteins, including Raplp, Abflp, ORC, and Ku70/80, bind to DNA and recruit a complex containing Sirep and Sir2p. The Sir2 protein then deacetylates histone substrates, perhaps most importantly lysine 16 of histone H4. This deacetylation event increases the affinity of SirSp for histones and is thought to create and propagate a heterochromatin-like structure refractory to the production of stable transcripts (reviewed in GREWAL and MOAZED 2003; RUSCHE et al. 2003). The S. cerevisiae Sir2 protein also has additional, less wellunderstood functions. These include repressing origin activation (PASERO et al. …

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