Cell Cycle Regulated Gene Expression in Yeasts - Advances in Genetics - page 63

and Wittenberg, 2009; de Bruin , 2006). Though it has been suggested that

the Cdc2p CDK activates MBF (Connolly

et al.

et al. , 1997; Reymond et al. , 1993),

other experiments indicate that it has no role (Baum , 1997) with, instead,

another cyclin, Pas1p, when associated with a kinase Pef1p, activating MBF
(Tanaka and Okayama, 2000). Other layers of controls also operate with, for
example, the cyclin Cig2p, which itself contains MCB sequences in its promoter
and whose transcription is at least in part controlled by MBF, inhibiting MBF
activity through binding to Res2p and phosphorylating Res1p when associated
with the CDK Cdc2p (Ayt ´

et al.

, 2001). Further controls may occur through
specific proteolytic degradation by the SCF ubiquitin ligase system during S, G2,
and M phases, when MBF is inactive (Yamano

et al.

et al. , 2000).

2. S-phase wave
A number of genes encoding histones, like the situation in budding yeast, are
periodically expressed in fission yeast during DNA synthesis (Matsumoto and
Yanagida, 1985; Matsumoto

, 1987), with the promoter motif and transcrip-
tion factor, Ams2p, that positively regulate their expression identified
(Takayama and Takahashi, 2007). Furthermore, a repressor named Hip1p has
been characterised, that is similar to the Hir1p/Hir2p repressors in budding yeast
(Blackwell

et al.

, 2004). Not all histone genes are expressed at the same cell cycle
time, with the centromere histone variant

et al.

cnp1 þ (CENP-A) expressed slightly
earlier in the cell cycle that the other histones, which is likely to be important for
function (Takahashi cnp1 þ is under different transcrip-

tional control mechanisms, though these remain unknown.

et al. , 2000). Presumably,

3. G2-M waves
Despite G2 contributing to a significant proportion of the mitotic cell cycle in
fission yeast, and the G2-M transition being the major control point, only a
smaller group of weakly regulated genes has been identified that are expressed at
this cell cycle stage. These include rds1 þ , with a putative

common promoter UAS proposed, named “New 3” (Rustici

spd1 þ , psu1 þ , and

, 2004). This
UAS has neither been confirmed experimentally nor has the transcription factor
that binds to it been identified.

et al.

4. M-G1 waves
An important wave of transcription occurs in fission yeast at the M-G1 interval.
This group of genes numbers at least 20 genes, with the first identified being
cdc15 þ (Anderson

et al. , 2002; Fankhauser et al. , 1995; Rustici et al. , 2004).