Transcriptional Control of the Plant Cell Cycle 25
3.3
3RMyb
Functional analysis of mitotic cyclin expression revealed that their periodic
accumulation at G2/M was regulated transcriptionally (Shaul et al. 1996; Ito
et al. 1997). Subsequent characterization of aCatharanthus roseusmitotic cy-
clin gene promoter led to the identification of a nine base pair sequence that
was sufficient to confer G2/M phase-specific stimulation of transcript accumu-
lation (Ito et al. 1998), which was designated the M-specific activator (MSA)
sequence. This sequence (YCYAACGGYYA, where Y is either T or C) has simi-
larities to the sequences that animal and plant Myb-type transcription factors
bind to. Using this sequence as a probe in a one-hybrid screen, three Myb-
type transcription factors were identified (Ito et al. 2001). In contrast to the
vast majority of plant transcription factors, which have only two of the canon-
ical Myb-repeats in their DNA binding domains, these factors have three such
repeats, just like the Myb genes in the animal kingdom (Ito et al. 2001).
The expression of one of these, NtmybB, was constitutive during the cell
cycle, whereas the transcripts of the other two, NtmybA1 and NtmybA2, ac-
cumulated periodically, commencing in late S-phase and peaking in G2 (Ito
et al. 2001). Using transfected tobacco BY2 cells to assay the ability of these
genes to affect expression of genes that accumulate maximally at G2/M, it
was shown that NtmybB represses, while NtmybA1 and NtmybA2 activate,
expression of such genes (Ito et al. 2001). Moreover, co-transfection assays
showed that these two classes of 3RMyb factors could compete for occupancy
ofcis-elements. Thus, the peak of MSA activity at the G2/M transition was
likely mediated by the transcriptional regulation of NtmybA1 and NtmybA2
factors. This suggests that the pronounced but transient activation of gene
expression conferred by the MSA element is the result of accumulation of
NtmybA-type activity against a constant background of NtmybB-mediated
repression, leading to a sharp maximum once this threshold is overcome (Ito
et al. 2001). Thus, the control of MSA-mediated gene expression at G2/M
by 3RMyb factors carries all the hallmarks of cell cycle-regulated gene ex-
pression. Interestingly, evidence for regulation of NtmybA2 by cyclin–CDK
complexes that effect a positive feedback loop was recently found: the non-
conserved activation domain of NtmybA2 was found to contain a repressor
domain, whose activity was relieved by CDK-dependent phosphorylation
(Araki et al. 2004). Only cyclins that are expressed at G2/M were able to
activate the cognate CDK complexes, but not cyclins that accumulate more
broadly during the cell cycle, suggesting that this feedback coupling is critical
for full-blown commitment to M-phase.
Recently, the putative Arabidopsis orthologs of NtmybA1 and NtmybA2,
MYB3R1 and MYB3R4, respectively, were characterized (Haga et al. 2007):
loss-of-function mutants in these genes are impaired in cytokinesis, often
resulting in multinucleate cells. Molecular analysis revealed that KNOLLE