Trends in Plant Science
OpinionDiversification of histone H2A variants during plant evolution
Section snippets
Histone variants
Eukaryotic genomes are compacted into chromatin, consisting of histone proteins and DNA. A basic unit of chromatin is the nucleosome, which is organized as ∼150 bp DNA wrapped around a histone octamer made from two of each core histone family, H2A, H2B, H3, and H4 1, 2. Between nucleosomes, the linker DNA may be bound by a member of the histone family H1 3, 4. In multicellular eukaryotes, histones are commonly encoded by multigene families, and individual paralogous genes of a histone family
Features defining H2A variants in flowering plants
To classify histone variants from non-flowering plants, we first analyzed H2A variants from a large set of flowering plants with the aim of defining variant-specific signatures (Figure S1 in the supplementary material online). In general, the lengths and specific amino acid sequences of C-terminal tails differentiate H2A proteins into distinct variant classes (Figure 1A). C-terminal tails of canonical H2A variants invariably end with a cluster of acidic amino acids, while C-terminal tails of
The diversity of histone variants in non-flowering land plants and green algae
Evolution of land plants is marked by at least three major transitions: (i) the conquest of the land by bryophytes, (ii) vasculature development and the dominance of diploid sporophytic generation in lycophytes, and (iii) the acquisition of elaborate reproductive structures including seeds and flowers in gymnosperms and angiosperms 49, 50, 51, 52. We used recently released genome sequences of key species that mark each of these transitions to identify histone-coding genes. H2A variants show a
Variants related to H2A.W
In the genome of the gymnosperm Picea abies we identified 13 variants, and these contained most of the signatures characteristic of H2A.W. Of those, five clearly cluster with flowering plant H2A.Ws (Figure 2). The other eight variants cluster as a separate group and differ from gymnosperm bona fide H2A.Ws in their C-terminal tails by the presence of the conserved sequence KSPKSK instead of KSPKKA (Figure 3). The L1 loop and the docking domain signatures of these variants resemble those of bona
Concluding remarks
Our survey across key species as representatives of plant evolution confirms that a large diversity of histone variants arose with the exception of H4. The lack of H4 variants is also notable in metazoans, and this strong evolutionary conservation of H4 sequence, together with the minimal sequence differences among major H3 variants (i.e., H3.1 and H3.3), likely reflects the high structural constraints required for the stabilization of the H3/H4 tetramer and its incorporation into nucleosome [1]
Acknowledgments
We are grateful to Paul Talbert for critical comments on the manuscript and members of the Berger lab for discussion of the data. We thank the Joint Genome Institute for providing the genomic assemblies and transcriptomes of the ongoing M. polymorpha genome project. This work was supported by Österreichische Akademie der Wissenschaften (ÖAW) and by a grant from the Österreichischer Fonds zur Forderung der wissenschaftlichen Forschung (FWF) (grant number P26887) to F.B.
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