G1/S Transition and the Rb-E2F Pathway 67
stopped cell division at an early stage of embryogenesis (Ronceret et al. 2005),
whereas weak mutation alleles caused lengthening of the cell cycle and af-
fected embryonic patterning (Jenik et al. 2005). In supporting that impaired
DNA replication/S-phase progression caused mutant phenotype, treatment
of wild-type embryos with the DNA replication inhibitor aphidicolin pheno-
copied the mutant phenotype (Jenik et al. 2005). The E2F-target geneRNR2,
encoding ribonucleotide reductase necessary for dNTP synthesis, is also re-
quired for normal S-phase progression (Wang and Liu 2006). Several studies
also revealed that DNA replication factors were involved in transcriptional
gene silencing (Elmayan et al. 2005; Kapoor et al. 2005; Inagaki et al. 2006; Xia
et al. 2006).
Newly synthesized DNA is immediately assembled with histones into chro-
matin (Fig. 2). The S-phase specific expression of histone genes is likely
controlled independently of the E2F family of transcription factors (Shen
and Gigot 1997; Meshi et al. 2000). Assembly of histones with DNA is me-
diated by different families of histone chaperones (Loyola and Almouzni
2004). Loss-of-function mutations ofFAS 1 andFAS 2,whichencodesub-
units of the CAF1 (Chromatin Assembly Factor1) family H3/H4 chaperones,
caused defects of meristems, resulting in shoot fasciation and root growth
inhibition (Kaya et al. 2001). The NAP1 (Nucleosome Assembly Protein1)
family genes, which encode H2A/H2B chaperones, are expressed to a rela-
tively constant level during different phases of the cell cycle (Dong et al.
2003). Post-translational modification by phosphorylation and regulated nu-
cleocytoplasmic shuttling of NAP1 proteins (Dong et al. 2005) likely provide
Fig. 2 Model of DNA replication and nucleosome assembly. Upon G1/S transition, tran-
scription of the E2F-target genes CDC6 and CDT1 results in firing of the DNA repli-
cation origin, through recruitment of MCM, DNA polymerases (Pol) and other less
well-characterized factors. Newly synthesized DNA is assembled with histones into nu-
cleosome, through action of histone chaperones (CAF1 and NAP1)