Abstract
Chromatin regulation of eukaryotic genomes depends on the formation of nucleosome complexes between histone proteins and DNA. Histone variants, which are diversified by sequence or expression pattern, can profoundly alter chromatin properties. While variants in histone H2A and H3 families are well characterized, the extent of diversification of histone H2B proteins is less understood. Here, we report a systematic analysis of the histone H2B family in plants, which have undergone substantial divergence during the evolution of each major group in the plant kingdom. By characterising Arabidopsis H2Bs, we substantiate this diversification and reveal potential functional specialization that parallels the phylogenetic structure of emergent clades in eudicots. In addition, we identify a new class of highly divergent H2B variants, H2B.S, that specifically accumulate during chromatin compaction of dry seed embryos in multiple species of flowering plants. Our findings thus identify unsuspected diverse properties among histone H2B proteins in plants that has manifested into potentially novel groups of histone variants.
Highlights
The basic subunit of chromatin is the nucleosome, which contains an octamer core of histones H2A, H2B, H3 and H4 wrapped around 147bp of DNA [1]
By characterizing H2Bs expressed in Arabidopsis somatic tissues, we identify a putative replacement histone H2B and reveal two groups with preferential deposition in heterochromatic and euchromatic regions of the genome
We noted a prominent split among eudicot H2B sequences which, with the exception of H2Bs from Asterids, fell into three major phylogenetic groups—Group 1 containing Arabidopsis H2B.1/2/4/9/11, Group 2 containing Arabidopsis H2B.5/6/7/10 and the angiosperm-specific group containing Arabidopsis H2B.8 (Fig 1B and 1C)
Summary
The basic subunit of chromatin is the nucleosome, which contains an octamer core of histones H2A, H2B, H3 and H4 wrapped around 147bp of DNA [1]. The tight control of nucleosomal organization is critical for chromatin processes like transcription, DNA replication, repair and recombination [2,3,4]. Individual paralogous genes of each histone family often encode related but functionally distinct proteins, which are referred to as “histone variants” when they acquire convergent properties during evolution [5, 6]. H2B histone variants provide new layers of chromatin organization and function
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