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The NAD + -dependent histone deacetylase Sir2 was originally identified in  Saccharomyces cerevisiae  as a silencing factor for  HML  and  HMR , the heterochromatic cassettes utilized as donor templates during mating-type switching.  MAT a cells preferentially switch to  MATα  using  HML  as the donor, which is driven by an adjacent  cis -acting element called the recombination enhancer (RE). In this study we demonstrate that Sir2 and the condensin complex are recruited to the RE exclusively in  MAT a cells, specifically to the promoter of a small gene within the right half of the RE known as  RDT1 . We also provide evidence that the  RDT1  promoter functions as a locus control region (LCR) that regulates both transcription and long-range chromatin interactions. Sir2 represses  RDT1  transcription until it is removed from the promoter in response to a dsDNA break at the  MAT  locus induced by HO endonuclease during mating-type switching. Condensin is also recruited to the  RDT1  promoter and is displaced upon HO induction, but does not significantly repress  RDT1  transcription. Instead condensin appears to promote mating-type donor preference by maintaining proper chromosome III architecture, which is defined by the interaction of  HML  with the right arm of chromosome III, including  MAT a and  HMR . Remarkably, eliminating Sir2 and condensin recruitment to the  RDT1  promoter disrupts this structure and reveals an aberrant interaction between  MAT a and  HMR , consistent with the partially defective donor preference for this mutant. Global condensin subunit depletion also impairs mating-type switching efficiency and donor preference, suggesting that modulation of chromosome architecture plays a significant role in controlling mating-type switching, thus providing a novel model for dissecting condensin function  in vivo .

A Sir2-regulated locus control region in the recombination enhancer of Saccharomyces cerevisiae specifies chromosome III structure