Figure 6.

Mapping of the histone–DNA contacts in a mono-prenucleosome. Core histones containing the wild-type or the indicated mutant histones were modified with OP, which links an o-phenanthroline moiety onto the histones via alkylation of the thiol group on cysteine residues. The resulting derivatized histones were reconstituted by salt dialysis into mono-prenucleosomes with the central 80 bp of 601 DNA that is ³²P-labeled at the 5′ end. The hydroxyl radical cleavage reactions were initiated by the addition of Cu(II), hydrogen peroxide, and mercaptopropionic acid. The cleavage products were purified and analyzed by electrophoresis on a 10% polyacrylamide–urea gel. The Maxam-Gilbert G+A ladder was used to identify the OP cleavage products, which are indicated at the right side of the autoradiogram. In a canonical nucleosome, H4S47C is located near the dyad.