Histone variants: are they functionally heterogeneous?

The basic subunit of eukaryotic chromatin is the nucleosome [1,2]. Two molecules of each of the core histone proteins - H2A, H2B, H3 and H4 - form an octamer, the protein component of the nucleosome core particle, around which 147 basepairs of DNA are wrapped. One histone molecule of the linker or H1 class binds to the octamer near the point where the DNA enters and exits the nucleosome and seals two full turns (approximately 166 basepairs) of DNA around the octamer [3]. Histone H1 also associates with linker DNA between nucleosomes to stabilize higher-order structures. As nucleosomal structure is similar in all metazoans, it is not surprising that histones are among the most highly conserved proteins in terms of both structure and sequence. But in higher organisms each histone subtype, with the possible exception of histone H4, is represented by a family of genes encoding multiple non-allelic primary-sequence variants [1,2,3,4,5]. Why should this be the case?There are several plausible explanations for the existence of multiple histone-encoding genes [4,5,6]. The first is simply gene dosage. A demand for high gene expression at specific times might require multiple active transcriptional units - for example, in the case of histones large amounts are needed during S phase when DNA is replicated and packaged into nucleosomes. In this case, heterogeneity at the protein-sequence level may be the result of genetic drift and would be of little consequence. An extension of this view might include heterogeneity at the level of regulation: multiple histone genes with distinct expression patterns during differentiation, in specific tissues, or under certain metabolic conditions might be necessary to ensure that adequate amounts of each histone are present in all cells. Evidence for this in higher organisms comes from the presence of replacement variants that, unlike most other histones, are expressed throughout the cell cycle and serve as a source of chromatin components nee