The histones

This chapter discusses evidence develops a hypothesis concerning the dynamic structural role of histones in chromatin. Histones are primarily structural proteins. Independent of species and tissue, they interact in the same repetitive fashion with the chromosomal DNA to form nucleosomes. Their lack of diversity and nonsequence-specific binding to DNA argue against their playing any explicit role in gene regulation. Histone microheterogeneity, arising from postsynthetic modifications and intraspecies sequence variations leaves open the possibility that histones are involved in the dynamics of chromosome structure. The chapter correlates the rather unique structural aspects of the five histone molecules—the differences among them as well as their similarities—with their biological function. A study of the many protein–protein and protein–DNA interactions in which the histones participate is discussed in the chapter. The four core histones H2A, H2B, H3, and H4 are focused, and H1 is discussed in relation to its interaction with DNA. The histones constitute part of a self-assembly system. And histone–histone interactions have a fundamental role in chromatin structure. Under conditions of high salt concentration, histone complexes related to the histone octamer—which is the histone core of the nucleosome—are obtained. Higher oligomeric structures are obtained from complexes of H2A·H2B, H3·H4, and all four histones. Intranucleosomal interactions of the histones are essential for the formation of the nucleosome. The basic general pattern of histone structure can be anticipated from the presence of a specific pattern of interactions of the core histones and the existence of histone variants and histone postsynthetic modifications.