Research

Frederic Berger

Histones, the basic proteins that wrap DNA into nucleosomes in eukaryotes, are commonly encoded by multigene families. Histones fall into five protein families, the core histones H2A, H2B, H3 and H4, and the linker histone family H1. A nucleosome core particle is made by assembling two proteins from each of the core histone families together with DNA. Linker DNA between core particles may be bound by a member of the H1 family. The individual paralogous genes of a histone family may encode related but distinct protein isoforms, commonly referred to as “histone variants”. Histone variants play critical roles in such diverse processes as transcription, chromosome segregation, DNA repair and recombination, chromatin remodeling and germline-specific DNA packaging and activation. During evolution of eukaryotes the number and complexity of variants had increased. In the kingdom Plantae, histone variants have multiplied from algae with a very low number of variants to flowering plants that show a remarkable high diversity of H3, H2A and H2B variants. Variants of H2A.Z and H3 have specialized functions, well studied in animals but much less is known of the functional specialization of most other histone variants and specific histone variants in plants. My group has shown the role of H3.3 in reprogramming the zygotic chromatin (Ingouff et al., 2010), and shown convergence of evolution of properties of variants between animals and plants (Wollmann et al 2012). We are currently developing a global approach of the role played by all histones variants in development and its relation to the environment combining structural analyses with functional analyses based on cellular, genetic and genomic strategies.

Fig. 1: Remodeling of parental H3 variants upon fertilization. Upon fertilization (A), the egg cell chromatin labeled with H3.3-GFP (green) fuses with the sperm cell chromatin labeled with H3.10-RFP (red) producing the zygote nucleus (arrowhead). The spreading of the paternal chromatin is observed in the nucleus of the fertilized central cell (fc). Between two hours (B) and four hours (C) after fertilization, H3.3-GFP provided by the female gamete and H3.10 provided by the sperm cell are removed and both GFP and RFP signals are barely detectable in the zygote nucleus (arrowhead). At eight hours after fertilization (D), the zygote produces H3.3-GFP de novo, that marks the zygote chromatin, (arrowhead) and the endosperm (end). (from Wollmann et al., 2012).
Fig. 2: H3.3 enrichment profile over genes correlates with expression and is biased towards the 3’ end. Average profile of H3.1 and H3.3 enrichment over the protein-coding genes grouped according to their expression levels into six different subsets (from the red to the purple curves corresponding to FPKM >30, 20-30, 10-20, 5-10, 1-5, 0-1, and containing 3179, 1463, 2897, 2344, 2780 and 1263 genes, respectively). H3.3 is enriched towards the end of gene bodies and positively correlated with levels of expression, whereas H3.1 enrichment over gene bodies is uniform. (from Ingouff et al., 2010).

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