Figure 2.

Identification of the ISW2 subunits that bind to different sites in the nucleosome. (A) The positions of the ribose residues protected from hydroxyl radical cleavage by ISW2 in the nucleosome are black (strong protection) or gray (weaker protection). The location of photoreactive nucleotides used to probe for ISW2 interactions within the nucleosome core particle is shown in red. Only one superhelical turn of DNA around the nucleosome is depicted. The epitope (R₁₇H₁₈R₁₉) in the H4 tail known to stimulate ATPase activity of ISWI is space filled in green. (B) Site-specific DNA photoaffinity labeling of ISW2 subunits. Nucleosomes (75-N-0) assembled with a series of photoreactive probes (222 bp) were incubated with or without ISW2 (72 nM, >90% of nucleosomes bound), UV-irradiated, and digested with DNase I and S1 nuclease as described. The probe position numbers indicate the site of incorporated photoreactive nucleotides with reference to the dyad axis. (C) Quantitative analysis of crosslinked ISW2 subunits Itc1 and Isw2 at each probe position is shown with the probe positions indicated below.