66 W.-H. Shen
additively repress expression of a reporter gene in a histone deacetylation
and ZmRpd3IRBR binding dependent manner. Rb-associated MSI proteins
are components of several protein-complexes involved in chromatin remod-
elling and play important roles in plant development (reviewed in Hennig
et al. 2005). Among these complexes, one contains the SET-domain pro-
tein CLF (Curly Leaf ) which itself can also directly bind RBR (Williams and
Grafi 2000) and is likely involved in histone H3 lysine 27 (H3K27) methyla-
tion (Chanvivattana et al. 2004; Jullien et al. 2006), implying a possible role
of H3K27 methylation in the Rb-E2F pathway. In humans, the SET-domain
H3K9-methyltransferase SUV39H1 binds to Rb and functions as a corepres-
sor of E2F activity with similarity to heterochromatin silencing (Nielsen et al.
2001). Overexpression of the SUV39H1 homolog in tobacco affected cell pro-
liferation but did not significantly perturb expression of the E2F-target gene
RNR2 (Shen and Meyer 2004; Yu et al. 2004). Plants contain a high number of
homologs of SUV39H1, however, their homologies are limited to the catalytic
region of the enzyme (Zhao and Shen 2004). Therefore, involvement of H3K9
methylation in the Rb-E2F pathway remains to be verified in plants.
4
DNA and Chromatin Replication
Initiation of DNA replication is licensed from multiple replication origins
once and only once per origin during each cell cycle (Diffley 2004). Ori-
gin firing at the G1/S transition occurs on the prereplication complexes,
composed of CDC6, CDT1, ORC (Origin Recognition Complex) and MCM
(MiniChromosome Maintenance) proteins that are conserved from yeast to
human (Fig. 1). InArabidopsis,bothCDC6andCDT1are E2F-target genes,
and CDC6 and CDT1 proteins are subjected to proteolysis control by the
proteasome (Castellano et al. 2001, 2004). CDT1 can be phosphorylated by
CYCD-CDKA and CYCA-CDKA, which likely triggers its proteolysis (Castel-
lano et al. 2004). Overexpression ofCDC6orCDT1induced endoreplication
(Castellano et al. 2001, 2004), likely because of promoted DNA replication li-
censing. Interestingly, down-regulation ofCDT1by RNAi impaired S-phase
progression, inhibited cell division as well as plastid division, but also in-
creased endoreplication (Raynaud et al. 2005). This increased endoreplication
could be due to elevated expression of MCM3 protein in the CDT1 RNAi
transgenic plants (Raynaud et al. 2005), nevertheless its precise mechanism
remains currently unknown. Loss-of-function mutations ofMCM7orORC2
were embryonic lethal (Holding and Springer 2002; Collinge et al. 2004).
Down-regulation by RNAi ofCDC45, which likely plays a role in recruitment
of DNA polymerase to the pre-replication complex, induced chromosome
fragmentation during meiosis, resulting in defective pollen and ovule devel-
opment (Stevens et al. 2004). Complete loss-of-function of DNA polymeraseε