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Distinct phases of Polycomb silencing to hold epigenetic memory of cold in Arabidopsis

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Science  15 Sep 2017:
Vol. 357, Issue 6356, pp. 1142-1145
DOI: 10.1126/science.aan1121

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Managing gene silencing through replication

Vernalization is the process in plants by which wintertime chill stimulates springtime flowering. Yang et al. and Jiang et al. show how chill is recorded in Arabidopsis epigenetically by methylation of histones. Specialized components of the Polycomb group of proteins remodel DNA to establish the methylation marks and are linked to DNA replication. Long-term stable epigenetic status follows rapid establishment of metastable epigenetic marks. This epigenetic strategy may be key to the developmental requirement of both secure and nimble fate decisions, allowing plant cells to change fates.

Science, this issue p. 1142 and p. 1146

Abstract

Gene silencing by Polycomb complexes is central to eukaryotic development. Cold-induced epigenetic repression of FLOWERING LOCUS C (FLC) in the plant Arabidopsis provides an opportunity to study initiation and maintenance of Polycomb silencing. Here, we show that a subset of Polycomb repressive complex 2 factors nucleate silencing in a small region within FLC, locally increasing H3K27me3 levels. This nucleation confers a silenced state that is metastably inherited, with memory held in the local chromatin. Metastable memory is then converted to stable epigenetic silencing through separate Polycomb factors, which spread across the locus after cold to enlarge the domain that contains H3K27me3. Polycomb silencing at FLC thus has mechanistically distinct phases, which involve specialization of distinct Polycomb components to deliver first metastable then long-term epigenetic silencing.

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