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. 2009 Feb 20;136(4):610-4.
doi: 10.1016/j.cell.2009.02.004.

Transcriptional scaffolds for heterochromatin assembly

Hugh P Cam  1 , Ee Sin Chen, Shiv I S Grewal
Affiliations

Transcriptional scaffolds for heterochromatin assembly

Hugh P Cam et al. Cell. .

Abstract

Heterochromatin is dynamically regulated during the cell cycle and in response to developmental signals. Recent findings from diverse systems suggest an extensive role for transcription in the assembly of heterochromatin, highlighting the emerging theme that transcription and noncoding RNAs can provide the initial scaffold for the formation of heterochromatin, which serves as a versatile recruiting platform for diverse factors involved in many cellular processes.

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Figures

Figure 1.
Figure 1.. Transcription and Heterochromatin Formation
Heterochromatin assembly in fission yeast requires coordinated function of histone-modifying enzymes (ClrC and histone deacetylases), HP1 proteins (Chp2 and Swi6), and the RNA interference (RNAi) machinery. RNAi factors include the Dicer enzyme (Dcr1), the RNA-induced transcriptional silencing (RITS) complex, and the RNA-dependent RNA polymerase complex (RDRC) that process centromeric repeat transcripts into siRNAs. (Top) During S phase of the cell cycle, the relatively open heterochromatic structure permits heightened RNA Pol II activity at centromeric repeats. This, in turn, stimulates the recruitment of heterochromatin-assembly factors such as the ClrC subunit Rik1 and the RITS subunit Argonaute 1 (Ago1), as well as histone H3 lysine 36 methylation by the Set2 methyltransferase implicated in the recruitment of the histone deacetylase (HDAC) silencing complexes such as Clr6 (Chen et al., 2008). Interaction between ClrC and RITS stabilizes their binding to chromatin and facilitates the processing of centromeric repeat RNAs to siRNAs (Zhang et al., 2008). Recruitment of ClrC may also be mediated by downstream siRNA products such as double-stranded RNAs. Methylation of lysine 9 on histone H3 (H3K9me) by the Clr4 subunit of ClrC not only recruits HP1 proteins but also establishes a positive feedback loop by stabilizing the chromatin association of ClrC (via Clr4 chromodomain) and RNAi components such as RITS (via Chp1 chromodomain). (Bottom) In G2 phase, HP1 proteins bound to H3K9me not only recruit silencing factors such as the HDAC complex SHREC but also an antisilencing factor Epe1 that promotes Pol II transcription. Spreading of HP1 proteins and H3K9me from the original nucleation sites allows heterochromatin to serve as a versatile recruiting platform for factors involved in diverse chromosomal processes. These include RNAi machinery that mediates posttranscriptional silencing in cis (cis-PTGS), HDACs involved in transcriptional gene silencing (TGS), and factors that are essential for genome stability.
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