Transcriptional Control of the Plant Cell Cycle 15
different mechanisms involved in cell cycle control contribute equally to its
overall regulation.
1.1
Oscillating Processes Involved in Cell Cycle Control
Three canonical mechanisms have been identified in cell cycle control of all
eukaryotic systems so far analyzed in sufficient detail: co-regulated and se-
quential expression of specific gene sets; cyclin–CDK mediated protein phos-
phorylation and proteasome-mediated proteolysis. Each of these processes
constitute a genuine, closed cycle such that periodic changes in activity states
promote, directly or indirectly, conditions that favor the following state, with
the “last” state promoting the first state of the series again, to form a closed
regulatory cycle. As each of these regulatory paradigms has recently been the
subject of excellent reviews, I shall restrict the discussion to an outline of the
general principles of the more universal mechanisms.
1.1.1
Transcriptional Networks in the Eukaryotic Cell Cycle
To review the logic of transcriptional control of cell cycle regulated gene
expression, our current understanding of such control in yeast is primarily
discussed here, as our current knowledge in plants is too incomplete. The
transcriptional networks in yeast that mediate periodic, cell cycle associated
gene expression are made up of at least nine serially regulated transcription
factors, most of which interact in various heteromeric complexes (Simon et al.
2001; Bähler 2005).
Periodic gene expression at the G1/S transition is mediated by two tran-
scription factor complexes, MBF and SBF, which accumulate and bind to
their target promoters from early G1 onwards. They remain inactive un-
til the Cln3p-Cdk1p cyclin–CDK complex that mediates entry into a new
cycle at START activates them in a two-step process. SBF and MBF bind
to the promoter of Swi4p, which is part of the SBF complex, thereby lead-
ing to a decisive, positive reinforcement of the decision to traverse START.
During S-phase, these complexes mediate transcription of S-phase-specific
genes including B-type cyclins required for progression into G2/M. In late
S- and G2-phase, SBF and MBF activity is down-regulated by CDK-mediated
phosphorylation that depends on cyclins transcribed earlier in the cycle by
SBF/MBF complexes.
The periodic expression of G2/M genes is governed by a ternary transcrip-
tion factor complex comprising Mcm1p; one of two partially redundant factors,
Fkh1p or Fkh2p, and Ndd1p. Ndd1p is the regulated component of the com-
plex, as the former two subunits stay bound to their targets throughout the
cell cycle. Ndd1p is regulated transcriptionally by SBF/MBF but also post-