FIG. 6.

rDNA chromatin accessibility of mutants as measured by in vivo psoralen cross-linking. (A) Log-phase cultures of lrs mutants were UV cross-linked with psoralen in vivo. Isolated DNA was digested with EcoRI and separated on a 1.3% agarose gel, and the transferred DNA was detected with a probe specific for the NTS of the rDNA (C). The sir2Δ strain (JS343) acted as a positive control for increased accessibility to psoralen cross-linking. Other strains shown are WT (JS306), top1 (M122), rif1 (M158), and cac1Δ (JS400). (B) An identical cross-linking procedure using the same NTS-specific probe was carried out on a subset of the irs mutants. The strains tested are WT (JS311), rpd3Δ (JS490), sin3Δ (JS493), sap30 (M475), hir3 (M411), and tup1 (M419). The control lanes show the 2.5-kb EcoRI fragment which is observed when the strains are not cross-linked. (C) Schematic drawing of the EcoRI rDNA fragment detected from this assay. ARS, autonomously replicating sequence.