Biology Reference
In-Depth Information
Notably, most of these genes encode products that have important and signifi-
cant roles in controlling progression through the cell division cycle, and many
are also periodically transcribed in humans by related transcriptional mechan-
isms (Cho
, 2002; Zwicker and Muller, 1995).
Among the waves of gene expression described in the two yeast species,
three show striking similarities between the two organisms. These are the SCB/
MCB waves at G1-S, the SFF/PCB waves at G2-M/M-G1, and the Swi5/Ace2
wave at M-G1.
In budding yeast, gene expression at G1-S is controlled by two tran-
scription factor complexes working in parallel, with some overlap in their
composition. SBF contains Swi4p and Swi6p, whereas MBF contains Swi6p
and Mbp1p. In fission yeast, MBF instead contains Cdc10p along with a number
of ancillary proteins. All three transcription factor complexes are related to each
other containing conserved structural motifs. Furthermore, the MCB DNAmotif
to which MBF binds is conserved between the two organisms. One significant
difference between the organisms, however, is that in fission yeast, there is just
one transcription factor complex operating at G1-S, which encompasses both
the functions of SBF and MBF. Despite these differences, related transcriptional
mechanisms operate in human cells with a wave of gene expression at G1-S
controlled by the E2F-Dp transcription factor complex (Genovese
et al.
, 2001; Whitfield
et al.
et al.
, 2006;
La Thangue and Taylor, 1993; Sherr and McCormick, 2002; Taylor
, 1997).
Other waves of gene expression occurring later in the cell cycle also
show similarities between the two yeasts and humans. These include those that
regulate various genes required late in the cell cycle for M phase and cytokinesis.
The waves of gene expression are all controlled by forkhead-like transcription
factors in both yeasts and humans (Alvarez
et al.
et al.
, 2001; Laoukili
et al.
, 2005; Park
et al.
, 2008). What is also striking is that these transcription factors control the
expression of and are controlled by polo kinases in both yeasts and humans
(Darieva
et al.
, 2006; Fu
et al.
, 2008; Papadopoulou
et al.
, 2008).
V. IMPORTANCE OF CELL CYCLE REGULATED GENE EXPRESSION
A significant question to consider is the importance of cell cycle regulated gene
expression. The fact that so many genes' transcription varies through the cell
cycle, and that in some cases this is conserved among organisms, suggests that
such regulation is required and important. Why is the expression of some genes
cell cycle regulated? A number of explanations have been suggested, which fall
into three broad categories: cell economy, impairment of cell cycle progression,
and cell cycle controls.
