FIG. 4.

Chromatin remodeling at HO involves the loss of canonical nucleosomes. (A) Schematic representation of the HO promoter. The solid black bars represent the probes used for Southern blot analysis. Nucleosome mapping involved preparing chromatin from the strain K8144 (ash1Δ SNF2) under conditions in which HO chromatin was in the repressed state (10 min following release) or the active state (25 min following release). The DNA fragment distribution of bulk chromatin could be seen by agarose gel electrophoresis followed by staining with ethidium bromide. (B) A ladder of nucleosomal repeats was detectable in chromatin from both the active and repressed conditions. To study the nucleosome contact at the HO promoter, fragments from agarose gels were transferred by Southern blotting and hybridized with a 722-bp probe spanning from bp −538 to −1260 within the HO URS region. (C) The loss of nucleosome length protection observed under activating conditions suggests that few nucleosomes remained capable of protecting 147 bp of DNA in this region. (D) Quantitative analysis of nucleosomal signal. Purified nucleosomal DNA was subjected to RT-PCR with primers specific for the nucleosome at bp −1000 to quantify loss of nucleosomal signal.