Biology Reference
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are usually stably silenced, individual cytosines are methylated in all contexts
with a typically higher level of methylation at CG than at CHG and CHH
sites (90%, 30-80%, and 30%, respectively) in somatic cells. By contrast,
methylation over protein-coding genes is almost exclusively found at CG sites
in the transcribed region and is associated with moderate expression rather
than silencing. Although conserved between plant orthologs (
Takuno &
Gaut, 2013
), functional role of gene-body methylation remains elusive as
its loss has only minor effects on expression (
Zilberman, Gehring, Tran,
Ballinger & Henikoff, 2007
).
Histones can be subject to a vast range of posttranslational covalent
modifications including acetylation, methylation, ubiquitinylation, or
sumoylation of specific residues, most of which are present in all eukaryotes
(Reviewed in
Kouzarides, 2007
). Epigenomic profiling in
Arabidopsis
has nota-
bly established that, as in other eukaryotes, trimethylation of histone H3 at
lysine 27 (H3K27me3) in plants is associated with gene repression and depends
on the evolutionary conserved Polycomb Repressive Complex 2 (PRC2)
(
Turck et al., 2007
;
Zhang, Clarenz et al., 2007
;
Bouyer et al., 2011; Lafos
et al., 2011; Schubert et al., 2006
). Four Polycomb-group proteins (PcG), orig-
inally identified in Drosophila, form the core of PRC2: the histone methyl-
transferase Enhancer of Zeste (E(Z)), Extra sex combs (ESC), Suppressor of
Zeste-12 (Su(Z)12), and the nucleosome remodeling factor 55 (NURF-55)
(
Margueron & Reinberg, 2011
). In
Arabidopsis
, with the exception of the
ESC homologue, FERTILIZATION-INDEPENDENT ENDOSPERM
(FIE), all other PcG components belong to protein families, suggesting
the existence of multiple PRC2 complexes. Thus, the
Arabidopsis
genome
encodes three homologues of E(Z), MEDEA, CURLY LEAF (CLF) and
SWINGER, five NURF-55-like proteins, named MULTICOPY SUP-
PRESSOR OF IRA1-5, and three Su(Z)12 homologues: FERTILIZA-
TION-INDEPENDENT SEED2 (FIS2), EMBRYONIC FLOWER2
(EMF2), and VERNALIZATION 2 (VRN2) (Reviewed in
Hennig &
Derkacheva, 2009
). Furthermore, at least three PRC2 complexes have been
distinguished in
Arabidopsis
based on the fact that FIS2 has a role during repro-
ductive development, whereas EMF2 and VRN2 control vegetative develop-
ment and are partially nonredundant (Reviewed in
Bemer & Grossniklaus,
2012; Hennig & Derkacheva, 2009
).
In
Drosophila
and mammals, additional PcG proteins form a second com-
plex, called PRC1, which participates in stabilizing PcG-mediated repres-
sion by binding to H3K27me3 and catalyzing the monoubiquitylation of
histone H2A (Reviewed in
Schuettengruber, Chourrout, Vervoort,
