Histone variants, nucleosome assembly and epigenetic inheritance

Template copying is the basis for inheritance and specification of genetic information and for the propagation of DNA methylation. However, no templating machinery has been identified that is capable of propagating histone modifications and conformations. Potential solutions to this enigma come from recent insights into histone variants and the specialized complexes that assemble them into nucleosomes. Different variants have been found to distinguish alternative chromatin states at centromeres, at the inactive mammalian X chromosome and at transcriptionally active loci. Remarkably, the histone variants H2A.Z and H3.3 have been found to participate in distinct nucleosome-assembly pathways, where H2A.Z is deposited specifically by the SWR1 complex, and H3.3 by the HIRA complex. For each of these pathways, the unit of assembly is a heterodimeric pair, which implies step-wise replacement of the nucleosomal octamer during transcription. The resulting replacement of modified histone variants might modulate barriers to transcription and perpetuate active chromatin states.