26 P. Doerner
(KN), a cytokinesis-specific syntaxin necessary for cell plate formation that
has MSA elements in its promoter, is under transcriptional control of 3RMyb
genes; this indicates that 3RMyb genes are crucial factors in a periodic gene
expression network involved in not only entry into G2/M but also key mech-
anisms of M-phase (Haga et al. 2007).
3.4
TCP
Expression of the proliferating cell nuclear antigen (PCNA) gene is cell cycle
regulated with a broad but shallow peak in S-phase (Menges et al. 2002),
consistent with its function as a processivity factor for DNA polymerase. Its
transcriptional regulation was first studied in rice which led to the discovery
of the proliferating cell nuclear antigen factors 1 and 2 (PCF1/2), the founding
members of a novel class of plant-specific group of transcription factors (Ko-
sugi and Ohashi 1997). Further in silico and experimental evidence showed
that the binding site for PCF1/2-type transcription factors (GCCCR, where
R is either G or A) was present in many additional genes associated with
growth and cell division (Tremousaygue et al. 2003; Li et al. 2005). Detailed
functional analysis of the Arabidopsis cyclin B1;1 promoter showed that the
GCCCR element was required only for the normal magnitude of expression,
but not for periodic, cell cycle phase-specific expression, which was in this
case conferred by the MSA element (Li et al. 2005). Moreover, the majority of
the genes with the cognateciselement in their promoter are not periodically
expressed, and those that are, are expressed in different phases. Together, this
indicates that while TCP genes contribute to the expression of cell cycle regu-
lated genes, they do not mediate periodic expression maxima.
3.5
Others
Many other transcription factor genes have been implicated in the expression
of cell cycle regulators. For example,AINTEGUMENTA(ANT), a member of
the plant-specific AP2 class of transcription factors, has been shown to stim-
ulate growth of leaf organs in Arabidopsis and, when over-expressed, leads to
increased expression of cyclin D3 (Mizukami and Fischer 2000). Likewise, the
relatedPLETHORA(PLT) transcription factors are implicated in the mainte-
nance of cell cycle gene expression (Aida et al. 2004). Similar functions are
ascribed toJAGGED(JAG) zinc-finger, KNAT-type homeodomain transcrip-
tion factors, and the novelULTRAPETALA(ULT) genes (Lincoln et al. 1994;
Ohno et al. 2004; Carles et al. 2005). However, a direct role in cell cycle gene
expression has not been demonstrated for any of these and presently, there
is no strong evidence that they are involved in periodically active cell cycle
transcription networks.