Fig. 5

Mechanistic basis for the role of HP1 proteins in gene regulation. A Within heterochromatin, HP1a promotes the formation of a condensed chromatin structure by binding to H3K9me2/me3 and forming polymers that bridge between adjacent polymers. This specialized chromatin structure represses expression of TEs and reporter genes but is required for the expression of genes native to this environment. B At heterochromatic piRNA clusters, the germline specific HP1 homolog Rhi binds to H3K9me2/me3 and together with Del and Cuff forms the RDC complex. The RDC complex then recruits Moon, which bypasses some of the steps required in euchromatin for RNA polymerase recruitment. Moon thus allows RNA polymerase recruitment to this chromatin environment and leads to transcription of the piRNA clusters. HP1a is also present and prevents transcription from other sites not targeted by RDC and Moon. C At some euchromatic genes, HP1a can be found at the promoter and interacting with heterologous ribonuclear proteins. It also interacts with RNA polymerase and the mRNA that is being produced. These interactions have an activating effect on transcription at these target sites. D At many euchromatic genes, HP1c is found in the promoter region together with the transcription factors Woc and Row. Together, they recruit Ubqn and Not, which promote the release of paused RNA polymerase. In addition, HP1c interacts with FACT, which promotes elongation by RNA polymerase. These two processes lead to transcriptional activation