Biology Reference
In-Depth Information
B. Fission yeast
In fission yeast, four main waves of gene expression have been described, with the
genes identified both by traditional methods and through transcriptome experi-
ments. These fall across various stages of the cell cycle, at G1-S, S phase, G2-M,
and M-G1 (Table 2.1; Fig. 2.2).
1. G1-S waves
The G1-S wave was the first to be identified in this organism and has been the
most well characterized. It contains up to 20 genes whose encoded products have
roles in DNA synthesis and cell cycle controls. Similar to that in budding yeast,
the genes all contain MCB sequence motifs in their promoters, which are the
UASs that control their G1-S-specific transcription (Lowndes
et al.
, 1992b;
Maqbool
, 2003). Indeed, the MCB DNA sequence motifs in budding and
fission yeasts are interchangeable, showing that this promoter motif is evolution-
arily conserved.
The transcription factor complex that binds to MCB sequences in
fission yeast is also called MBF (originally DSC1), with a number of components
identified. First among these was Cdc10p, which appears to have a central role in
controlling G1-S gene expression with, like Swi6p, both positive and negative
functions (Lowndes
et al.
, 1995). Cdc10p is similar to
Swi4p and Swi6p in peptide sequence, predicted structure, and also contains
ankyrin motifs, suggesting a common ancestry for this class of transcription factor
(Aves
et al.
, 1992b; McInerny
et al.
, 1985). Other components of MBF include Res1p (also named
Pct1p), Res2p, Rep1p, and Rep2p, each with apparently differing functions,
but altogether contributing to ensure the correct expression at the beginning
of S phase, with some having mitotic and others meiotic specific roles (Ayt´
et al.
et al.
, 1995, 1997; Baum
et al.
, 1997, 1998; Caligiuri and Beach, 1993; Miyamoto
et al.
, 1994; Tahara
et al.
, 1998; Tanaka
et al.
, 1992; White
et al.
, 2001;
Whitehall
, 1994). For example, Rep2p has an important
regulatory role in controlling MBF activity in mitosis (Nakashima
et al.
, 1999; Zhu
et al.
, 1995);
and Res2p and Rep1p have significant roles in controlling meiotic gene expres-
sion (Cunliffe
et al.
, 2004; Ding and Smith, 1998). Other proteins to which
Cdc10p bind have been identified, and include Pol5p, which controls rRNA
production (Nadeem
et al.
et al.
, 2006), and the MBF complex also has a role in DNA
damage response (Chu
, 2007).
HowMBF is itself controlled is not fully understood, although this is not
through cell cycle-specific binding to promoter DNA, at least of Cdc10p, as it
contacts MCB motifs throughout the cell cycle (Wuarin
et al.
, 2002). As
mentioned, Cdc10p has both repressing and activating roles (McInerny
et al.
,
1995), with further repressive functions conferred to MBF by SpNrm1p (de Bruin
et al.
