Biology Reference
In-Depth Information
When cells progress through the cell cycle, a very large number of
different cellular structures and processes are made or activated, in many cases
for a short period to last only for the cell cycle phase for which they are required.
For example, during the S phase when DNA replication occurs, proteins required
for making DNA must be present, and during M phase proteins needed for
chromosome separation need to be made. Therefore, an efficient and economical
way to use cellular constituents would be to produce proteins only at the cell
cycle stage when they are required, using regulated gene transcription as one
mechanism among others to achieve this.
In other instances, it may be deleterious for cell cycle progression for
certain proteins to be present at a particular cell cycle phase when they are
usually not present. Such proteins may inhibit or prevent cell cycle progression,
or result in faulty processes occurring. Cell cycle-specific expression is one way of
ensuring that proteins are only present at the cell cycle phase when they are
meant to be present. Examples of this are the
cdc18 þ gene in fission yeast, and the
cdc18 þ is a critical controller of S
phase, being a component of the origin of replication complex (ORC) that is
under MCB-MBF control and normally only transcribed at G1-S (Kelly
BUD8
and
BUD9
gene pair in budding yeast.
,
1993). Its artificial expression at other times in the cell cycle causes multiple S
phases which are lethal to cells. Indeed, the restriction of the presence of Cdc18p
to the start of S phase is so critical to cells that it is controlled at multiple levels,
including transcription and protein stability (Jallepalli
et al.
, 1997). The Bub8p
and Bub9p proteins are related membrane proteins that control localization of
cell poles, partly through their expression at different cell cycle times
(Schenkman
et al.
, 2002). This was shown by the swapping of their promoters
resulting in each gene product being produced at the wrong cell cycle time and
consequent mis-localization to the opposite pole.
Another interesting function for cell cycle-specific expression is when it
plays a role in regulating or controlling cell cycle progression. For this small
number of genes, their phase-specific expression is crucial for controlling the
timing of, and the orderly, correct passage into the next cell cycle phase. A good
example of this is shown by the polo kinases, Plo1p and Cdc5p, protein kinases
with central roles in controlling M-phase progression and cytokinesis, conserved
between the two yeast species, and also present in humans (Archambault and
Glover, 2009). In all three eukaryotes, the genes are cell cycle regulated, and this
regulation is very important, as its deregulation has serious effects. The most
dramatic example of this is shown in fission yeast where the expression of
et al.
plo1 þ
during interphase results in premature and lethal cytokinesis (Ohkura
,
1995). This is in part because Plo1p controls its own and other genes' transcrip-
tion at the M-G1 interval by directly binding to promoters and phosphorylating
at least one component of the PBF transcription factor complex that controls M-
G1 gene expression in this organism (Anderson
et al.
et al.
, 2002; Ng
et al.
, 2006;
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