Histone variants meet their match

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Abstract
Chromatin is organized into specialized domains, some of which contain specialized histones called variant histones. Variant histones have evolved to carry out functions that are distinct from those of the major core histones. Histones are deposited onto DNA by chaperones. Recent studies have shown that some chaperones recruit specific histone variants for deposition. The chaperones HIRA and SWR1 specifically recruit and exchange the histone variants H3.3 and H2AZ, respectively, whereas the chromatin assembly factor-1 (CAF1) mediates the deposition of the H3.1 major core histone in a process that is coupled to DNA replication. The implications of two modes of deposition of the histones H3 and H4 (that is, tetramers versus dimers) are discussed, as they are important for the transmission of epigenetic information from the mother to the daughter cells. The functions and possible modes of deposition of other variant histones such as CENPA, H2AX and others are also discussed. Both H3.3 and H2AZ have been suggested to function in activated transcription on the basis of their localization to euchromatic loci. Models are proposed whereby histone exchangers coordinate with FACT — a chaperone that has a crucial role in facilitating transcription elongation on chromatin — to allow the incorporation of histone variants during the process of transcription. A model is also presented that addresses CAF1-mediated deposition of histone H3 that is methylated at Lys9, through CAF1 interaction with the histone methyltransferase SUV39H1, which results in the formation of repressive chromatin.