Establishment of heterochromatin in domain-size-dependent bursts

Methylation of histone H3K9 is a hallmark of epigenetic silencing in eukaryotes. Nucleosome modifications often rely on positive feedback where enzymes are recruited by modified nucleosomes. A combination of local and global feedbacks has been proposed to account for some dynamic properties of heterochromatin, but the range at which the global feedbacks operate and the exact mode of heterochromatin propagation are not known. We investigated these questions in fission yeast. Guided by math-ematical modeling, we incrementally increased the size of the mating-type region and profiled heterochromatin establishment over time. We observed exponential decays in the proportion of cells with active reporters, with rates that decreased with domain size. Establishment periods varied from a few generations in wild type to >200 generations in the longest region exam-ined, and highly correlated silencing of two reporters located outside the nucleation center was observed. On a chromatin level, this indicates that individual regions are silenced in sud -den bursts. Mathematical modeling accounts for these bursts if heterochromatic nucleosomes facilitate a deacetylation or methy-lation reaction at long range, in a distance-independent man -ner. A likely effector of three-dimensional interactions is the evolutionarily conserved Swi6HP1 H3K9me reader, indicating the bursting behavior might be a general mode of heterochromatin propagation.

Automated summary

The study found that heterochromatin establishment shows exponential decays, with establishment periods varying from a few generations to over 200 generations. This indicates individual regions may be silenced in sudden bursts on a chromatin level, potentially suggesting a general mode of heterochromatin propagation.