Fluorescently labelled histones as probes of nucleosome structure. Preparation and general properties of methionine-labelled histone H4.

A fluorescent derivative of calf thymus histone H4 has been prepared by the reaction of methionine-84 with N-(iodoacetylaminoethyl)8-naphthylamine-1-sulfonic acid at pH 2.4 in 8 M urea. The preparation and characterization of this labelled histone is described. Fluorescence emission measurements indicate that the label on H4 undergoes a 3--5-fold increase in emission intensity when H4 self-interacts or binds to DNA alone or is incorporated in a synthetic nucleosome. The changes observed are consistent with the formation of varied apolar environments around methionine-84, due most likely to histone-histone rather than histone-DNA interactions. Preliminary experiments indicate that the precise emission intensity of labelled H4 in the nucleosome is quite sensitive to conditions of ionic strength and histone integrity.