Histone variants: are they functionally heterogeneous?

Abstract

In most eukaryotes, histones, which are the major structural components of chromatin, are expressed as a family of sequence variants encoded by multiple genes. Because different histone variants can contribute to a distinct or unique nucleosomal architecture, this heterogeneity can be exploited to regulate a wide range of nuclear functions, and evidence is accumulating that histone variants do indeed have distinct functions.

Figure 1

Potential mechanisms of H1-mediated regulation of transcription. (a) Linker histones stabilize and/or promote the formation of transcriptionally inert higher-order chromatin structures. (b) The removal or modification of the binding of histone H1 to relaxed chromatin structures may be a prerequisite for initiating a series of events resulting in transcriptional activation. These events may include covalent modification (for example acetylation) of core histones, chromatin remodeling by sliding or removal of nucleosome cores, and the establishment of a transcription preinitiation complex. Differences among H1 variants in their ability to promote or antagonize any of these transitions could contribute to transcriptional regulation.