Local chromatin context dictates the genetic determinants of the heterochromatin spreading reaction
AbstractHeterochromatin spreading, the expansion of gene-silencing structures from DNA-encoded nucleation sites, occurs in distinct chromatin contexts. Spreading re-establishes gene-poor constitutive heterochromatin every cell cycle, but also invades gene-rich euchromatin de novo to steer fate decisions. Unlike heterochromatin nucleation and assembly, the determinants of the spreading process remain poorly understood. Our heterochromatin spreading sensor separately records nucleation site-proximal, and distal, heterochromatin gene silencing. By screening a nuclear function gene deletion library in fission yeast, we identified regulators that alter the propensity, both positively and negatively, of a nucleation site to spread heterochromatin. Critically, the involvement of many regulators is conditioned by the chromatin context within which spreading occurs. We find spreading, but not nucleation, within constitutive heterochromatin, requires distinct Clr6 histone deacetylase complexes. However, spreading is universally antagonized by a suite of chromatin remodelers. Our results disentangle the machineries that control lateral heterochromatin spreading from those that instruct DNA-directed assembly.