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Leblanc, & Cavalli, 2007; Simon & Kingston, 2009
). In addition to this
sequential interaction, PRC2 and PRC1 can also act redundantly and inde-
pendently of each other (
Leeb et al., 2010; Tavares et al., 2012
). Although
PRC1 components are less well conserved over evolution (
Schuettengruber
et al., 2007
), several lines of evidence indicate that plants also have PRC1-
like complexes. Thus, three homologues of BMI1 (AtBMI1A, B, and C) and
two homologues of RING1 (AtRING1A and B) have been identified in
Arabidopsis
and are involved in gene repression. In addition, EMBRYONIC
FLOWER 1 and LIKE HETEROCHROMATIN PROTEIN1 (LHP1)
interact with RING1 and BMI1 homologues, and LHP1 has been proposed
to fulfill a function analogous to Polycomb (Pc), for which there is no
bona fide
homologue in plants (
Bratzel, Lopez-Torrejon, Koch, Del
Pozo, & Calonje, 2010; Chen, Molitor, Liu, & Shen, 2010; Kim, Lee,
Eshed-Williams, Zilberman, & Sung, 2012; Li et al., 2011; Mylne et al.,
2006; Turck et al., 2007; Xu & Shen, 2008; Zhang, Germann et al., 2007
).
In
Arabidopsis
, between 15% and 30% of all genes are targeted by PRC2,
depending on the tissue or organ analyzed (
Lafos et al., 2011; Turck et al.,
2007; Zhang, Clarenz et al., 2007
). H3K27me3 marking differs considerably
between meristematic and differentiated cells, suggesting dynamic recruitment
of PcG proteins (
Lafos et al., 2011
). Only a limited subset of PRC2 targets
appears to depend on PRC1 for stable transcriptional repression (Bratzel
et al., 2010). This indicates that, like in mammals, Pc-based regulation in
Ara-
bidopsis
does not necessarily imply cooperation between the two complexes. As
in other eukaryotes, PcG-mediated repression is antagonized by the activity of
JumonjiC domain-containing proteins that demethylates H3K27me3 (
Lu,
Cui, Zhang, Jenuwein, & Cao, 2011
) and by proteins of the Trithorax group
(TrxG). TrxG proteins include the histone methyltransferase
ARABIDOPSIS
TRITHORAX1 (ATX1) (
Alvarez-Venegas et al., 2003
), which catalyzes
trimethylation of H3K4me3 and the SAND domain protein
ULTRAPETALA1 (ULT1) (
Carles & Fletcher, 2009
). Two SWI2/SNF2
chromatin remodeling ATPases, SPLAYED (SYD) and BRAHMA (BRM),
also contribute to Trx function (
Smaczniak et al., 2012; Wu et al., 2012
).
2.2. Epigenome organization in plants
Despite the bewildering variety of chromatin marks and chromatin-
associated proteins, large-scale epigenomic studies in several eukaryotes
including
Caenorhabditis elegans
,
Drosophila
,
Arabidopsis
, and human cells iden-
tified a limited combinatorial repertoire of chromatin modifications
corresponding to a relatively small number of chromatin states (Reviewed
