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In-Depth Information
(see
Table 8.2
). Furthermore, there is a group of three PHD-finger
proteins—
VRN5, VIN3
, and
VEL1
, which are considered as functionally
related to Pcl because they enhance the KMT activity of the complex
(
Lucia, Crevillen, Jones, Greb, & Dean, 2008
). As expected from the exis-
tence of numerous paralogs, there are multiple PRC2 complexes, which
are involved in different processes. The development of the female
gametophyte and the initiation of embryogenesis are controlled by the
MEA-FIS2-MSI1-FIE complex and mutation in any of the four genes is
embryonically lethal (
Grossniklaus et al., 1998; K¨hler et al., 2003; Luo
et al., 1999; Ohad et al., 1999
). The PRC2 maternal effect phenotype is cau-
sed by imprinting defects in the endosperm (functionally analogous to the
mammalian placenta) and derepression of MADS box TFs, which leads to
hyperproliferation of the endosperm and eventually seed abortion. Further-
more, by expressing a FIE transgene in
fie
mutant plants, it was possible to
overcome the maternal requirement for PRC2 and identify pleiotropic phe-
notypes as the result of derepression of
KNOX
homeobox genes (
Katz,
Oliva, Mosquna, Hakim, & Ohad, 2004
). Another PRC2 variant consisting
of CLF/SWN-EMF2/VRN2-MSI1-FIE is involved in the regulation of
several key transcription factors (AG, FLC), which control the transition
from vegetative-to-reproductive development and the cold-induced
flowering response (
Coustham et al., 2012
; reviewed in
Holec & Berger,
2012; Song, Angel, Howard, & Dean, 2012
).
3.1.2 PRC1
3.1.2.1 Drosophila
The
Drosophila
PRC1 consists of Sce, Pc, Psc, and Ph and similar to PRC2
has been implicated in the
Hox
gene regulation (see
Table 8.3
). The expres-
sion of PRC1 genes throughout fly development resembles the one of
PRC2 and mutations lead to classical homeotic phenotypes and embryonic
lethality (
Breen & Duncan, 1986; Dura, Brock, & Santamaria, 1985; Dura
et al., 1987; Graveley et al., 2011; J¨ rgens, 1985; Lewis, 1947, 1978;
McQuilton et al., 2012
). More recently, Sce and Psc have been identified
as members of a distinct complex, dRING-associated factors (dRAF), con-
taining also the H3K36-demethylase dKDM2 (
Lagarou et al., 2008
). The
kdm2
mutant allele significantly enhances the
Pc
homeotic phenotype and
in the same time rescues the
Trx
and
Ash1
mutations (ASH1 and TRX
are H3K36- and H3K4-specific KMTs, respectively). Biochemical analysis
revealed that dRAF but not PRC1 is the major complex involved in the
ubiquitination of H2AK119, and this activity is directly linked to the
removal of the H3K36me2 PTM. This is a striking example of how
