Biology Reference
In-Depth Information
3.1.2.3 Plants
The composition of PRC1 complexes in
Arabidopsis
has not been studied in a
systematic way and to the depth as in mammalian systems or in
Drosophila
,
mostly because the PRC1 homologues have been identified only recently
in
Arabidopsis
(
Sanchez-Pulido, Devos, Sung, & Calonje, 2008
). A plant
PRC1-like complex has been characterized by
Xu and Shen (2008)
, showing
interaction between AtRING1a/AtRING1b and the H3K27me3-binding
CD protein LHP1 (
Turck et al., 2007
). While single
AtRING1a
or
AtRING1b
homozygous mutants did not show any abnormal phenotype,
double mutant plants displayed homeotic transformations and meristem stem
cell phenotypes similar to the ones observed in
lhp1
/
and
clf
/
plants
(
Gaudin et al., 2001; Goodrich et al., 1997; Larsson, Landberg, & Meeks-
Wagner, 1998
).The loss of
AtRING1a/b
caused derepression of
KNOX
genes promoting meristem proliferation, whereas H3K27me3 levels at the
promoters of affected genes did not change, which shows that PRC2 function
was still intact but transcriptional repression requires AtRING1a/b (
Xu &
Shen, 2008
). The latter observation was confirmed in a study by Bratzel
and colleagues who examined the role of
AtBMI1a
and
AtBMI1b
, two of
the three identified Pcgf homologues in
Arabidopsis
(
Bratzel et al., 2010;
Sanchez-Pulido et al., 2008
). Plants deficient for both
AtBMI1a
and
AtBMI1b
showa variety of phenotypes, ranging fromcomplete developmen-
tal arrest of early seedlings to more mild effects related to cell differentiation
and formation of callus-like structures. This phenotypes correspond to the
observed upregulation of stem cell regulators such as
WUS, STM, FUS3,
LEC1
, and
WOX5
. Furthermore, Bratzel and coworkers identified for the
first time that
Arabidopsis
PRC1 proteins AtBMI1a/B and AtRING1a/b
are involved in monoubiquitination of H2A.1 (the
Arabidopsis
H2A homo-
logue that retained the lysine substrate at position 121) and are associated in a
complexwith LHP1 and a nonconserved plant-specific protein EMF1. Inter-
estingly, EMF1 has been implicated with a dual role as being part of both
PRC1 and PRC2 complexes in
Arabidopsis
and repressing two independent
sets of genes (
Kim, Lee, Eshed-Williams, Zilberman, & Sung, 2012
). Plants
depleted of
emf1
show homeotic and flowering phenotypes (
Aubert et al.,
2001
), as well as derepression of PRC2 targets like AG, FLC, and SEP1-3
but not the imprinted
PHE
genes.
3.2. Polycomb regulation
As evident from the many examples given above, PcG proteins are required
throughout the life cycle of eukaryotes: during embryogenesis, for somatic
