Abstract
BackgroundToxoplasma gondii is a protozoan parasite that differentiates from acute tachyzoite stages to latent bradyzoite forms in response to environmental cues that modify the epigenome. We studied the distribution of the histone variants CenH3, H3.3, H2A.X, H2A.Z and H2B.Z, by genome-wide chromatin immunoprecipitation to understand the role of variant histones in developmental transitions of T. gondii parasites.ResultsH3.3 and H2A.X were detected in telomere and telomere associated sequences, whereas H3.3, H2A.X and CenH3 were enriched in centromeres. Histones H2A.Z and H2B.Z colocalize with the transcriptional activation mark H3K4me3 in promoter regions surrounding the nucleosome-free region upstream of the transcription start site. The H2B.Z/H2A.Z histone pair also localizes to the gene bodies of genes that are silent but poised for activation, including bradyzoite stage-specific genes. The majority of H2A.X and H2A.Z/H2B.Z loci do not overlap, consistent with variant histones demarcating specific functional regions of chromatin. The extent of enrichment of H2A.Z/H2B.Z (and H3.3 and H2A.X) within the entire gene (5’UTR and gene body) reflects the timing of gene expression during the cell cycle, suggesting that dynamic turnover of H2B.Z/H2A.Z occurs during the tachyzoite cell cycle. Thus, the distribution of the variant histone H2A.Z/H2B.Z dimer defines active and developmentally silenced regions of the T. gondii epigenome including genes that are poised for expression.ConclusionsHistone variants mark functional regions of parasite genomes with the dynamic placement of the H2A.Z/H2B.Z dimer implicated as an evolutionarily conserved regulator of parasite and eukaryotic differentiation.
Highlights
Toxoplasma gondii is a protozoan parasite that differentiates from acute tachyzoite stages to latent bradyzoite forms in response to environmental cues that modify the epigenome
Localization of histone variants is not uniform within nuclei We confirmed the location of histones variants in the nuclear compartment by labeling intracellular RH strain tachyzoites with monospecific polyclonal antibodies recognizing T. gondii histone variants or endogenous epitope-tagged proteins, and imaging by deconvolution fluorescence microscopy (Fig. 1)
Comparison of localization patterns with the staining intensity of DAPI allowed us to examine the presence of the histone variants in dense and less dense chromatin
Summary
Toxoplasma gondii is a protozoan parasite that differentiates from acute tachyzoite stages to latent bradyzoite forms in response to environmental cues that modify the epigenome. Eukaryotic cells have specialized macromolecular complexes that organize and pack DNA inside the nucleus. These multiprotein complexes include histones (H2A, H2B, H3, H4) and non-histone proteins. The interaction of histones with DNA and non-histone proteins affects all nuclear processes, including replication, Nardelli et al BMC Genomics (2022) 23:128 transcription and DNA repair. Histones are extensively modified with posttranslational modifications (PTMs). These PTMs regulate DNA-related processes in a positive or negative way, influencing the interaction between the histones and DNA or recruiting protein factors involved in gene expression or gene silencing. Unlike canonical histones, whose synthesis is coupled to DNA replication during S-phase, histone variants are typically constitutively expressed through the cell cycle [6]. The physical properties of histone variants can change chromatin structure, or histone variants may localize to specific regions of the genome to confer specialized properties to chromatin
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