Heterochromatin and RNAi Are Required to Establish CENP-A Chromatin at Centromeres

Heterochromatin is defined by distinct posttranslational modifications on histones, such as methylation of histone H3 at lysine 9 (H3K9), which allows heterochromatin protein 1 (HP1)-related chromodomain proteins to bind. Heterochromatin is frequently found near CENP-A chromatin, which is the key determinant of kinetochore assembly. We have discovered that the RNA interference (RNAi)-directed heterochromatin flanking the central kinetochore domain at fission yeast centromeres is required to promote$\text{CENP}-\text{A}^{\text{Cnp}1}$and kinetochore assembly over the central domain. The H3K9 methyltransferase Clr4 (Suv39); the ribonuclease Dicer, which cleaves heterochromatic double-stranded RNA to small interfering RNA (siRNA); Chp1, a component of the RNAi effector complex (RNA-induced initiation of transcriptional gene silencing; RITS); and Swi6 (HP1) are required to establish$\text{CENP}-\text{A}^{\text{Cnp}1}$chromatin on naïve templates. Once assembled,$\text{CENP}-\text{A}^{\text{Cnp}1}$chromatin is propagated by epigenetic means in the absence of heterochromatin. Thus, another, potentially conserved, role for centromeric RNAi-directed heterochromatin has been identified.