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Review
. 2012 Jul;20(5):521-34.
doi: 10.1007/s10577-012-9288-x.

Centromeric heterochromatin assembly in fission yeast--balancing transcription, RNA interference and chromatin modification

Benjamin J Alper  1 , Brandon R Lowe, Janet F Partridge
Affiliations
Review

Centromeric heterochromatin assembly in fission yeast--balancing transcription, RNA interference and chromatin modification

Benjamin J Alper et al. Chromosome Res. 2012 Jul.

Abstract

Distinct regions of the eukaryotic genome are packaged into different types of chromatin, with euchromatin representing gene rich, transcriptionally active regions and heterochromatin more condensed and gene poor. The assembly and maintenance of heterochromatin is important for many aspects of genome control, including silencing of gene transcription, suppression of recombination, and to ensure proper chromosome segregation. The precise mechanisms underlying heterochromatin establishment and maintenance are still unclear, but much progress has been made towards understanding this process during the last few years, particularly from studies performed in fission yeast. In this review, we hope to provide a conceptual model of centromeric heterochromatin in fission yeast that integrates our current understanding of the competing forces of transcription, replication, and RNA decay that influence its assembly and propagation.

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Figures

Fig. 1
Fig. 1
Propagation of centromeric heterochromatin during DNA replication. Heterochromatin is characterized by H3K9me which serves to recruit RNAi components and Clr-C. During S-Phase, DNA replication dilutes the H3K9me mark and allows access to pol II, which leads to the appearance of marks of active transcription such as H3K4me3, H3K36me3, and H3K4Ac. The H3K4Ac reduces Chp1 binding affinity for H3K9me2/3, effectively reducing RITS levels associated with chromatin. This allows the lower affinity Swi6 and Chp2 to bind H3K9me and recruit chromatin modifying complexes containing deacetylases such as Clr3 and Clr6 which promote the removal of the H3K4Ac mark. It is likely that Clr4 is recruited though Rik1 associated with the elongating DNA polymerase Cdc20, allowing re-accumulation of the H3K9me mark. Concurrent with transcription, centromeric transcripts are converted into siRNA through the actions of RDRC and Dcr1. RITS is then recruited to the centromeric heterochromatin though Ago1’s binding of siRNA and interactions with Clr-C. RITS then recruits more Clr-C to further propagate the centromeric heterochromain
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