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Comment
. 2011 Mar 15;25(6):529-33.
doi: 10.1101/gad.2038611.

RNAi genes pave their own way

Leonie M Kamminga  1 , René F Ketting
Affiliations
Comment

RNAi genes pave their own way

Leonie M Kamminga et al. Genes Dev. .

Abstract

Heterochromatin formation in fission yeast and the role of RNAi in this process have been intensively studied. So far, however, nothing is known about the regulation of expression of RNAi components during these events. Gullerova and colleagues (pp. 556-568) reveal an autoregulatory loop that regulates the expression of RNAi genes and centromeric heterochromatin formation during the cell cycle. Gene orientation plays a surprising role in this process.

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Figures

Figure 1.
Figure 1.
Models for RNAi at pericentromeric and CG loci in S. pombe. (A) At the pericentromeric loci, an Argonaute protein-containing complex named RITS targets nascent Pol II transcripts and recruits an RDRC. RDRC synthesizes dsRNA that is processed by Dicer (Dcr1) into siRNAs that can associate with Ago1 in the context of the ARC complex. This results in the formation of another RITS complex. RITS, via the protein Stc1, also recruits the CLRC complex. CLRC promotes H3K9me, which in turn recruits HP1 homologs like Swi6. Heterochromatin can spread to adjacent sites by further recruitment of chromatin modifiers like SHREC. (B) At CG loci, bidirectional transcription of CGs leads to dsRNA formation and subsequent siRNA generation by Dcr1. RDRC does not play a significant role in RNAi at CG loci. Further mechanisms appear to be similar to those in A. Black arrows indicate CGs. White arrows in Pol II indicate direction of transcription.
Figure 2.
Figure 2.
RNAi during the cell cycle. An overview of the autoregulatory loop of convergent RNAi genes and heterochromatin formation at the centromeres during the different stages of the cell cycle is depicted. During mitosis, H3S10 gets phosphorylated, resulting in the displacement of Swi6 with condensin. Both CGs and pericentromeric repeats are at this time covered by H3K9me and condensin. At centromeres, Swi6 remains partly present. During G1/S, condensin delocalizes, leading to transcriptional activity. RNAi processes are triggered at both CG and centromeric loci. As the cells move into G2, the accumulated cohesin at CG loci starts to prevent dsRNA formation, and repression of CGs is relieved. These CGs include many RNAi component genes, resulting in a stronger RNAi response and further heterochromatin formation and transcriptional silencing at the centromeres.
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