Figure 4.

Acetylated chromatin is DNase I sensitive. (A) DNase I analysis of control and acetylated chromatin. Reconstituted chromatin was digested with increasing concentrations of DNase I (see Materials and Methods), and the deproteinized DNA fragments were analyzed on an agarose gel. A negative print of the ethidium-stained DNA is shown. (B) Left, The superhelical density of the two types of chromatins is identical. Acetylated and control chromatin was reconstituted at histone:DNA (wt/wt) ratios between 2 and 4. Deproteinization leaves an unrestrained supercoil for each nucleosome. Plasmid topoisomers were resolved on an agarose gel in the presence of chloroquin. The plasmid isolated from bacteria, at a superhelical density of 0.05 (69), served as a marker (M) equivalent to ≈31 superhelical turns. An equivalent superhelicity on plasmids in chromatin corresponds to about one nucleosome per 197 bp. Right, DNase I-sensitive chromatin is not sensitive to MNase. The MNase analysis was as in Fig. 2. (C) The preferential DNase I sensitivity of acetylated chromatin is enhanced in the presence of H1. Chromatin was reconstituted from control and acetylated histones in the presence of H1 (see Fig. 2) to an equivalent degree (not shown) and digested with DNase I as in A.