MINIREVIEW Reinventing Heterochromatin in Budding Yeasts: Sir2 and the Origin Recognition Complex Take Center Stage

The transcriptional silencing of the cryptic mating-type loci in Saccharomyces cerevisiae is one of the best-studied models of repressive heterochromatin. However, this type of heterochromatin, which is mediated by the Sir proteins, has a distinct molecular composition compared to the more ubiquitous type of heterochromatin found in Schizosaccharomyces pombe, other fungi, animals, and plants and character-ized by the presence of HP1 (heterochromatin protein 1). This review discusses how the loss of important heterochromatin proteins, including HP1, in the budding yeast lineage presented an evolutionary oppor-tunity for the development and diversification of alternative varieties of heterochromatin, in which the conserved deacetylase Sir2 and the replication protein Orc1 play key roles. In addition, we highlight how this diversification has been facilitated by gene duplications and has contributed to adaptations in lifestyle. In Saccharomyces cerevisiae, repressive chromatin forms at the cryptic mating-type loci, HMRa and HML, and pre-vents the expression of extra copies of the genes that deter-mine mating-type (reviewed in reference 99). These two loci enable mating-type switching but must remain silenced t