Histone variants and sensing of chromatin functional states

Abstract

An extreme case of chromatin remodelling is the genome-wide exchange of histones with basic non-histone DNA-packaging proteins that occurs in post-meiotic male germ cells. The scale of this genome reorganization is such that chromatin needs to undergo a prior "preparation" for a facilitated action of the factors involved. Stage-specific incorporation of specialized histone variants, affecting large domains of chromatin, combined with histone post-translational modifications accompany the successive steps of the male genome reorganization. Recently, it has been shown that a testis-specific H2B variant, TH2B, one of the first identified core histone variants, replaces H2B at the time of cells' commitment into meiotic divisions and contributes to the process of global histone removal. These investigations also revealed a previously unknown histone dosage compensation mechanism that also ensures a functional interconnection between histone variant expression and histone post-translational modifications and will be further discussed here.

Keywords: acetylation; arginine methylation; crotonylation; histone chaperone; histone degradation.

Figure 1. A model for chaperone-mediated sensing of chromatin states. Chaperone-histone interaction upon nucleosome assembly recruits transcriptional activators/repressors modifying the state of gene expression as a function of the amount of assembled histone.

Figure 2. A model for variant-dependent histone modifications by the PTM machinery within chaperones. The example of the TH2B-H2AL2 dimer is used to illustrate the hypothesis that the forced association of H2AL2 with H2B instead of TH2B, would render certain H2B positions accessible to the PTM machinery.