Histone positions within the nucleosome using platinum labeling and the scanning transmission electron microscope

An approach to studying the organization of macromolecular complexes using heavy-atom labeling has been developed and applied to the problem of determining the positions of the histone proteins within the nucleosome. The approach is based on the capability of the scanning transmission electron microscope to image heavy atoms. Nucleosomes containing histones labeled with heavy atoms were prepared by lysine modification of selected histones with methyl (methylthio)acetimidate, followed by reconstitution of the modified histones into nucleosomes, and reaction of the reconstituted nucleosomes with chloroglycyl-1-methioninatoplatinum (II). Micrographs of the platinum-labeled nucleosomes were obtained using the scanning transmission electron microscope, and analyzed using both computer and manual techniques. The results of the analysis were 24 A resolution maps of the distribution of high electron scattering density picture elements (representative of platinum atoms) indicating the position of each histone. The significance of those results and the general applicability of the platinum-labeling techniques are discussed. Finally, a description of the histone positions within the nucleosomes is presented and discussed in relation to the current literature on nucleosome structure.