FIG. 4.
Hypoacetylated and hyperacetylated nucleosomes are cross-linked with the same efficiency in 100 mM NaCl. 32P-labeled 180-bp probe DNA was reconstituted in nucleosomes with control, hypoacetylated, or hyperacetylated histones (17 acetyl groups per histone octamer). Samples, in a solution of 100 mM NaCl, were irradiated with single 266-nm laser pulses at different intensities, and the yield of cross-linked DNA was measured by the phenol extraction procedure. (A and B) Dose-response curves for hypoacetylated (A) and hyperacetylated (B) nucleosomes. (C) Acid-urea-Triton gel electrophoresis of hypoacetylated (lane 1) and hyperacetylated (lane 2) histones used for reconstitution. The number of acetylated groups is indicated by numbers 0 to 4. (D) Immunoslot assay of the reaction of antibodies to H2A, H2B, and H4 and preimmune IgG (0) with covalent histone-DNA complexes isolated after centrifugation in CsCl gradients of irradiated nucleosomes containing hypoacetylated (a) and hyperacetylated (b) histones; c, immunoslot assay of the material from control (hypoacetylated), nonirradiated particles after centrifugation in CsCl. The experiment was carried out as described in the legend to Fig. 3B.