Biology Reference
In-Depth Information
This dependency between two waves of gene expression is the only example in
fission yeast, but is strikingly similar to that seen in budding yeast (B¨hler,
2005a). It has been suggested that it is part of the mechanism that allowed fission
to evolve into a single-celled organism (B¨hler, 2005b; Borup, 2006).
III. REGULATORY NETWORKS
Now that reasonably comprehensive pictures of cell cycle regulated gene expres-
sion in both budding and fission yeasts have been created, attempting to link
together the various waves to each other in each organism, and to integrate these
into more general cell cycle controls has been possible.
In budding yeast, a combination of traditional methods and genome-
wide approaches has created an integrated “cycle within a cycle” whereby
the consecutive waves of gene expression are functionally linked to one another
with, in the simplest example, one wave of gene expression containing
the transcription factor that controls transcription of the next wave of gene
expression (Fig. 2.3; Breeden, 2003; Ihmels
et al.
, 2002; Lee
et al.
, 2002; Simon
et al.
, 2001; Tyers, 2004; Wittenberg and Reed, 2005). Arbitrarily beginning the
cell cycle at G1-S, SBF and MBF control expression of
HCM1
, with
Hcm1p controlling the expression of
and with Ndd1p activating the
Fkh2p-Mcm1p complex, which, in turn, stimulates transcription of
NDD1
SWI5
and
ACE2
. Swi5p and Ace2p, along with Mcm1p, stimulate the production of both
Swi4p and Cln3p which, either directly or indirectly, activate SBF and MBF.
Thus, the loop is completed. On top of this “loop of transcription” is
another layer of posttranslational control, with certain gene products activating
or repressing the activity of transcription factor complexes (de Lichtenberg
et al.
, 2007).
In contrast, such a “loop of transcription” appears in fission yeast not to
be complete (Fig. 2.3). So far, the only direct link between two consecutive cell
cycle waves of transcription occurs with the Ace2p transcription factor being
under transcriptional control by PBF at M-G1, with it controlling the Ace2
wave of gene expression (B¨hler, 2005a; McInerny, 2004; Nachman and Regev,
2009). Consistent with the suggestion that the other waves of gene expression
are unlinked, deregulation of PBF-PCB gene expression has no effect on MBF-
MCB controls, and vice versa (Anderson
, 2002). Instead, it appears that
most waves are controlled separately by cell cycle molecules such as the CDK
Cdc2p or the polo kinase Plo1p. Quite how the global mechanism will operate
in higher eukaryotes, and whether a loop of transcription operates, remains
to be seen.
et al.
