Role of histone tyrosines in nucleosome formation and histone-histone interaction

We have studied the functional properties of iodinated histones. Isolated, denatured histones were iodinated at trace levels and then renatured together with carrier histones and high molecular weight DNA to form nucleohistone. Nucleosomes were prepared from the reconstitute using micrococcal nuclease, and the relative representations of the individual iodinated tyrosines of the histones in the reconstituted nucleosomes were determined. Our principal findings are 1) that denatured histones can be iodinated at any tyrosine without interfering in subsequent nucleosome reconstitution and 2) that the resulting reconstituted nucleosomes nevertheless possess histone cores of altered stability, being either more or less stable depending on the particular tyrosine which is iodinated. We show that tyrosines 37, 40, and 42 of H2B are protected from iodination in intact core particles, as expected since these tyrosines lie within the H2B-H2A binding site. Yet iodination of these tyrosines in denatured H2B does not interfere with nucleosome assembly. However, the histone cores isolated from these reconstituted nucleosomes are of diminished stability as assayed by Sephadex column chromatography in 2 M salt. In contrast, iodination of tyrosines 83 and 121 of H2B, as well as iodination of the tyrosines of H2A, increases the stability of the histone octamer core. Iodination of H4 tyrosine 72 is without effect on histone octamer stability. Tyrosine iodination constitutes a profound amino acid alteration in the context of the absolute evolutionary conservation of most histone tyrosines. For example, all H2Bs sequenced to date, from fungi to mammals, possess tyrosines at positions 37, 40, and 42. Our results suggest that the immutability of these tyrosines reflects some sophisticated function of the nucleosome histone core beyond the assembly and mere maintenance of a compact structure.