Direct evidence for SIR2 modulation of chromatin structure in yeast rDNA

The yeast SIR2 gene maintains inactive chromatin domains required for transcriptional repression at the silent mating-type loci and telomeres. We previously demonstrated that SIR2 also acts to repress mitotic and meiotic recombination between the tandem ribosomal RNA gene array (rDNA). Here we address whether rDNA chromatin structure is altered by loss of SIR2 function by in vitro and in vivo assays of sensitivity to micrococcal nuclease and dam methyltransferase, respectively, and present the first chromatin study that maps sites of SIR2 action within the rDNA locus. Control studies at the MAT alpha locus also revealed a previously undetected MNase-sensitive site at the a1-alpha 2 divergent promoter which is protected in sir2 mutant cells by the derepressed a1-alpha 2 regulator. In rDNA, SIR2 is required for a more closed chromatin structure in two regions: SRR1, the major SIR-Responsive Region in the non-transcribed spacer, and SRR2, in the 18S rRNA coding region. None of the changes in rDNA detected in sir2 mutants are due to the presence of the a1-alpha 2 repressor. Reduced recombination in the rDNA correlates with a small, reproducible transcriptional silencing position effect. Deletion and overexpression studies demonstrate that SIR2, but not SIR1, SIR3 or SIR4, is required for this rDNA position effect. Significantly, rDNA transcriptional silencing and rDNA chromatin accessibility respond to SIR2 dosage, indicating that SIR2 is a limiting component required for chromatin modeling in rDNA.