Circadian Regulation of Cell Division 9
pressing the expression of TOC1. On the other hand, TOC1 is responsible for
the activation of CCA1/LHY, though probably indirectly, since there is a de-
lay between the peak of TOC1 expression and the transcription of CCA1/LHY.
This picture of the plant circadian clock is far from being complete, since this
simple loop does not account for several experimental observations (Locke
et al. 2005). In particular TOC1 overexpression led to an unpredicted de-
crease in CCA1 (Hayama and Coupland 2003). Furthermore,cca1/lhydouble
mutants still exhibit rhythmicity (Mizoguchi et al. 2002). A second loop was
recently identified by a mathematical modelization approach. A predicted
candidate Gigantea (GI) is also induced by light leading to the activation of
TOC1and is transcriptionally repressed by CCA1 (Locke et al. 2005). Though
the molecular roles of the respective actors remain to be precisely assessed,
this two-loop model accounts much better for experimental data, than the
one-loop model does.
The question then is, how does the circadian clock regulate cell division
in plants? In hepatocytes, the central CLOCK-BMAL1 complex binds the E
box response element (CACGTG) resulting in a direct activation ofWEE
transcription (Matsuo 2003). InArabidopsis,cis-acting elements mediating
gene expression have been identified in the promoters of circadian-regulated
genes (ref ). The EE (AAAATATCT), mediates the transcriptional repression of
TOC1by CCA1 at the heart of the clock. A single mutation in the EE to the
AAAAATCT CBS sequence (CCA1 binding site element) switches the phase of
catalase expression from evening to morning (Michael and McClung 2002).
We have performed a genome-wide analysis of the main core cell cycle gene
promoters inArabidopsis(Table 1). Both EE and CBS sequences were iden-
Table 1In silicoidentification of potential circadian elements (EE and CBS) inArabidopsis
core cell cycle genes
Gene Accession Motif Location Function
number
CDKA,1 At3g48750 AAATATCT (EE) – 1130 G1/S G2/M
CDKB1,2 At2g38620 AAATATCT (EE) – 200 G2/M
AAAAATCT (CBS) – 300
CDKD,2 At1g66750 AAAATATCT (EE) – 570 G2/M
cyclin D3,1 At4g34160 AATATCT (EE) – 280 G1/S
cyclinA3,1 At5g43080 AAAATATCT (EE) – 1000 S/M?
AAAAATCT (CBS) – 1800 S
DEL2 At5g14960 AAAATATCT (EE) – 20 G1/S
Retinoblastoma At1g22803 AAATATCT (EE) – 500
AAAATCT (CBS) – 500
KRP2 At3g50630 AATATCT (EE) – 1600 G2/M
KRP3 At5g48820 AAATATCT (EE) – 2200 G1/S G2/M?