What Holds Epigenetic Memory?

RESEARCH HIGHLIGHTS

Nature Reviews Molecular Cell Biology | Published online 21 Feb 2017; doi:10.1038/nrm.2017.15

the repressed FLC state. These factors in the same cells were used to Journal club promote cold-induced changes in the investigate the cis memory storage chromatin state at FLC, specifically a capability of FLC chromatin. After WHAT HOLDS EPIGENETIC MEMORY? localized increase in trimethylated cold treatment, expression of the two Lys27 of histone 3 (H3K27me3) reporters was independent; in other How epigenetic memory is stored and covering nucleosomes +3, +4 and +5. words, one of the FLC reporters could inherited is an on-going debate This peak then ‘spreads’ upon return be repressed in the same cell in which (Ptashne). In so-called ‘cis memory’, epigenetic to warmth, and high levels of the other reporter was active. epigenetic information is proposed to memory of H3K27me3 accumulate to cover the Furthermore, this ‘mixed’ expression be physically stored in local whole FLC locus, which is associated state was stably inherited through chromatin states that are associated, FLC repression with long-term epigenetic silencing. multiple cell divisions, indicating that for example, with DNA methylation is physically Taken together, these results the epigenetic memory of FLC or histone modifications. By contrast, located in suggested a cis-based epigenetic repression is physically located in the in ‘trans memory’, epigenetic the local memory of cold, with inheritance of local chromatin environment information is stored in the the silenced state occurring (Berry et al.). The debate is over for concentration of a diffusible factor chromatin independently at each locus. Polycomb-mediated silencing - local such as a transcriptional repressor. environment However, as in other model chromatin states can hold memory. An elegant natural system in epigenetic systems, until recently it Caroline Dean which to study this issue is the was difficult to exclude the existence John Innes Centre, cold-induced epigenetic silencing at of trans factor-based memory in Norwich Research Park, Norwich NR4 7UH, UK Arabidopsis thaliana FLOWERING which the FLC protein (a MADS box [email protected] LOCUS C (FLC). FLC mRNA and transcriptional repressor) and/or The author declares no competing interests. protein decrease quantitatively non-coding RNA produced at FLC with increasing cold exposure, as feed into a trans-regulatory network. ORIGINAL ARTICLES Ptashne, M. On the use of the word ‘epigenetic’. Curr. Biol. 17, R233–R236 FLC becomes repressed in an A study stimulated by theoretical (2007) | Angel, A. et al. A Polycomb-based switch increasing number of cells over time considerations of epigenetic underlying quantitative epigenetic memory. Nature 476, 105–108 (2011) | Berry, S. et al. Local (Angel et al.). regulation provided the definitive chromatin environment of a Polycomb target PHD-Polycomb repressive proof for cis memory (Berry et al.). gene instructs its own epigenetic inheritance. complex 2 components are required Two distinguishable fluorescent eLife 4, e07205 http://dx.doi.org/10.7554/ eLife.07205 (2015) for the induction and maintenance of reporters of FLC expression present