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Kloc A, Zaratiegui M, Nora E, Martienssen R. RNA interference guides histone modification during the S phase of chromosomal replication // Curr Biol. 2008 Apr 8;18(7):490-5.
Cold Spring Harbor Laboratory, Cold Spring Harbor, New York 11724, USA.
BACKGROUND: Heterochromatin is chromosomal material that remains condensed throughout the cell division cycle and silences genes nearby. It is found in almost all eukaryotes, and although discovered (in plants) almost 100 years ago, the mechanism by which heterochromatin is inherited has remained obscure. Heterochromatic silencing and histone H3 lysine-9 methylation (H3K9me2) depend, paradoxically, on heterochromatic transcription and RNA interference (RNAi). RESULTS: Here, we show that heterochromatin protein 1 in fission yeast (Swi6) is lost via phosphorylation of H3 serine 10 (H3S10) during mitosis, allowing heterochromatic transcripts to transiently accumulate in S phase. Rapid processing of these transcripts into small interfering RNA (siRNA) promotes restoration of H3K9me2 and Swi6 after replication when cohesin is recruited. We also show that RNAi in fission yeast is inhibited at high temperatures, providing a plausible mechanism for epigenetic phenomena that depend on replication and temperature, such as vernalization in plants and position effect variegation in animals. CONCLUSIONS: These results explain how "silent" heterochromatin can be transcribed and lead to a model for epigenetic inheritance during replication.
PMID: 18394897 [PubMed - indexed for MEDLINE]

Comment on:
Curr Biol. 2008 Apr 8;18(7):490-5.
Nature. 2008 Feb 7;451(7179):734-7.
Histone modifications: cycling with chromosomal replication.
Thon G.
Department of Biology, University of Copenhagen, BioCenter, Copenhagen N, Denmark.
Histone modifications tend to be lost during chromosome duplication. Several recent studies suggest that the RNA interference pathway becomes active during the weakened transcriptional repression occurring at centromeres in S phase, resulting in the re-establishment of histone modifications that direct the formation of heterochromatin.
PMID: 18460317 [PubMed - indexed for MEDLINE]