Biology Reference
In-Depth Information
II. CELL CYCLE GENE EXPRESSION IN YEASTS
A. Budding yeast
In budding yeast, traditional and genome-wide experiments have identified a
relatively large number of genes whose transcript levels fluctuate significantly
through the cell cycle. The two original transcriptome experiments identified up
to 800 genes, with approximately 300 strongly periodically expressed in both
studies (Cho
, 1998), though subsequent high-
resolution mapping has identified more, with up to 1100 genes (Pramila
et al.
, 1998; Spellman
et al.
et al.
,
2006; Rowicka
, 2007). These analyses described periodic genes that fall into
groups expressed at four cell cycle stages: those at the start of S phase, others at
the G1-S transition, more during S phase, and those at the start and the end of M
phase, the G2-M and M-G1 transitions (Table 2.1; Fig. 2.2).
et al.
1. G1-S waves
One of the first mechanisms of cell cycle gene expression in yeasts to be identified
whose transcription mechanisms are well characterized was a group of two waves
of genes whose peak transcript levels occur at the so-called
, the
start of S phase, in budding yeast (Breeden, 1996). This gene expression occurs at
a time in the cell cycle where cells decide whether to commit to cell division or
to exit from the cell cycle, which is called Start in yeasts, or the restriction point
in mammalian cells.
In budding yeast, the two waves of gene expression are controlled by two
different, but overlapping, transcription factor complexes, named MCB-binding
factor (MBF; also sometimes called DSC1), and SCB-binding factor (SBF). MBF
is composed of at least two proteins, Swi6p and Mbp1p, whereas SBF contains
Swi6p and another protein, Swi4p (Andrews and Herskowitz, 1989a,b; Dirick
et al.
G1-S transition
, 1992). In
each case, the transcription factors bind to different short DNA sequence motifs,
sometimes repeated, present in the promoters of the genes that they control,
which are the UASs. One of the motifs is called an MCB, for
, 1992; Koch
et al.
, 1993; Lowndes
et al.
, 1991, 1992a; Primig
et al.
Mlu
I cell cycle box
as, coincidentally, it is identical to the
Mlu
I restriction enzyme recognition site
(McIntosh
, 1991); and the other motif is called an SCB, for Swi4/Swi6-
dependent cell cycle box. It is believed thatMbp1p and Swi4p have DNA-binding
properties, whereas Swi6p, in both cases, is thought instead to have a regulatory
role. All three proteins share sequence similarities with each other, including
ankyrin motifs thought to be required for protein-protein interactions, suggesting
that they evolved from a common ancestor (Bork, 1993).
Broadly speaking, SBF and MBF control the expression of groups of
genes with different functions at G1-S. SBF-controlled genes function mostly in
cell wall biosynthesis
et al.
and cell budding, whereas MBF targets
genes
