Fig. 7

Model of stage-specific gene regulation by histone variants. Model of different nucleosomal composition on T. gondii chromatin. Different chromatin regions have distinct configurations that differentially change the chromatin architecture according to DNA-related processes. Nucleosomes in typical heterochromatin regions such as centromeres contain the specific centromeric histone CenH3, together with H2A.X. H3.3 enrichment is also evident in those regions, suggesting an atypical heterodimer with CenH3 and H3.3 or alternate CenH3 and H3.3-containing nucleosomes. H2A.X likely forms a tetramer with an unknown H2B, probably H2Ba. Canonical H2Ba with H2A.X is located at opposite sites to H2A.Z/H2B.Z dimer. H3.3 and H2A.X are also enriched at chromosome ends. Transcription start sites (TSS) of active genes are enriched with the variant dimer H2A.Z/H2B.Z. On the CDS of these genes, H2A.Z/H2B.Z are replaced with H2A.X (with H2Ba), and the canonical histone H3 by H3.3. Transcriptionally silenced genes have a similar composition with H2A.Z, H2B.Z and H3.3 at the TSS. But this configuration extends throughout the CDS of these genes, especially at genes which are specific for bradyzoites. These regions do not contain H3K4me3, a landmark for active transcription, suggesting that these genes could be poised. H4 has no variants, so it should be present throughout the genome