FIGURE 1 Proposed model for heterochromatin formation at the centromere in S. pombe. RITS is the RNA-induced transcriptional silencing complex, RDRC is the RNA-directed RNA polymerase complex, and ARC is an Argonaute chaperone complex that contains duplex siRNA. Synthesis of dsRNA and generation of siRNA occur in association with specific chromosome regions. The nascent transcript model proposes that the RITS complex mediates heterochromatin formation by associating with nascent transcripts via siRNA base pairing, and with methylated H3K9 (histone H3 with methylated Lys⁹). RNA-directed association of RITS with specific transcripts (step 1) leads to RNA-dependent RNA copying (step 2) and processing (step 3) to increase the level of small RNAs that can bind to ARC and associate with RITS to continue the cycle (step 4).