Functional separation of the requirements for establishment and maintenance of centromeric heterochromatin

The establishment and maintenance of centromeric heterochromatin in fission yeast require the RITS complex. Comprised of centromeric siRNAs, the chromodomain protein Chp1, Argonaute (Ago1), and Tas3, RITS couples the cellular RNAi pathway with assembly of constitutive heterochromatin. However, the mechanisms governing RITS-dependent establishment versus maintenance of centromeric heterochromatin remain unresolved. Here, we report that a mutant Tas3 protein that cannot bind Ago1 supports the maintenance of centromeric heterochromatin but cannot mediate efficient de novo establishment from cells transiently depleted for the histone H3 lysine 9 methyltransferase Clr4. In contrast, centromeric heterochromatin efficiently assembles in mutant cells transiently depleted for dicer. This mutant therefore allows ordering of the events leading to establishment of centromeric heterochromatin and places lysine 9 methylation of histone H3 upstream of dicer function.