In line with previous observations, histone modifications associated with active genes are conserved, though some some interesting exceptions exist, specifically regarding broad histone acetylation domains and H3K79 methylation. 5/n
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Histone acetylation associated with genes is typically restricted to active gene TSSs, but in three distantly related species, an ichthyosporean, a chytrid fungus, and a flowering plant, we saw elevated levels of histone acetylation along the length of inactive or lowly expressed genes. 6/n
H3K79 methylation is written by a single enzyme, DOT1, and is associated with active gene bodies in fungi and animals. Surprisingly, while we saw dimethylation positively correlated with gene expression in the amoebazoan A. castellanii, trimethylation was negatively correlated! 7/n
(Here I went down a bit of a literature rabbit hole trying to understand how this could happen, but thanks to some great work in the trypanosome T. brucei, we hypothesize that the sub- and neofunctionalization of Dot1 homologs may be the key!) 7.5/n
Dearest to me is the diversity of histone modification combinations found in repressed heterochromatin. Traditionally, constitutive heterochromatin on transposable elements was defined by H3K9 methylation, and facultative heterochromatin on silent genes was defined by H3K27me3. 8/n
My favourite example is from the chytrid fungus S. punctatus, which has a near perfect colocalization of H3K9me3 and H3K27me3 encompassing both TEs and genes (reminiscent of the effect of the dual writing ability of Ezl1 in ciliates) 10/n
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