Biology Reference
In-Depth Information
with these having important roles in controlling general gene expression, and
their correct relative stoichiometric levels are important for correct chromosome
transmission to daughter cells (Meeks-Wagner and Hartwell, 1986). Thus, their
periodic expression is important and the transcription factors that regulate this
need to be identified.
In fission yeast, a group of genes was identified by transcriptome analysis
that is expressed at the G2-M interval, with a putative promoter sequence, “New
3” proposed from computer analyses (Rustici
, 2004). This sequence neither
has been confirmed experimentally nor has the transcription factor complex been
identified, although it has been suggested that the transcript levels of some of the
genes may be controlled by changes in mRNA stability (B¨hler, 2005a).
et al.
B. Integration into general cell cycle controls
For both yeast species, frameworks suggesting how the various waves of genes are
controlled through the cell division cycle have been created. At least for budding
yeast, this framework describes a transcription cycle within a cycle, though this is
not the case for fission yeast, where the cycle is incomplete. Even so, in both
yeasts, the waves of transcription need to be integrated and controlled by general
cell cycle control mechanisms, to ensure that expression of the various waves is
coupled to cell cycle processes for which they are relevant.
In some cases, such links have been established (Fig. 2.4). Perhaps, the
most important cell cycle control molecules are the CDKs, which in yeasts are
represented by single proteins, Cdc28p in budding yeast and Cdc2p in fission
yeast. Both CDKs are sequentially activated through the cell cycle, by binding to
different cyclin molecules to stimulate and control various cell cycle stages, such
as S phase and M phase. As such, the cyclin-CDK molecules have a preeminent
role in controlling cell cycle events, and in some cases they appear to directly
link these to transcriptional regulation, of which a few examples have been
described in this review, in both yeasts, at various stages of the cell cycle.
Additionally, it has been shown that CDKs have roles in more general transcrip-
tional regulation in both budding and fission yeasts (Lee
,
2003). However, global approaches have demonstrated that, at least in budding
yeast, CDKs do not control all cyclic waves of gene expression, so other regulator
molecules must be involved (Orlando
et al.
, 2005; Morris
et al.
, 2008).
One such class of molecules is the polo kinases, which have a major
function in controlling processes late in the cell cycle. Indeed, at least in fission
yeast, it is known that once cells have passed the G2-M transition and entered
mitosis, CDK activity must be inhibited to allow cytokinesis and cell division to
occur (Le Goff
et al.
, 1999). In both yeasts and humans, it has been shown that
polo kinases directly control cell cycle gene expression at this late cell cycle stage
et al.
