Biology Reference
In-Depth Information
Fkh2p, and Sep1p (Buck
,
1997, 1999). So, not only manipulating individual genes but also altering the
expression of whole groups of genes can have little effect. In a few cases, this can
be explained by genetic redundancy, with double deletions of transcription factor
pairs having far more severe phenotypes. But it is a fact that the removal of
apparently important cell cycle transcription factors affecting whole groups of
target genes has, in some cases, very little effect.
However, at least two important points need to be considered in such
discussions about the relevance of cell cycle gene expression. The first is that
most, if not all, experiments studying this phenomenon in yeasts, are with cells
growing under optimal conditions during exponential growth phase, and so going
through multiple, consecutive cell cycles. It is likely that these are not usual
growth conditions that yeast experience in the wild where, instead, they spend
most time in stationary phase, with short bursts of division and growth, when
nutrients sporadically appear. This means that perhaps more relevant to the
biology of these single-celled organisms is their first divisions immediately after
exiting from stationary phase. Under such conditions, the G1 phase is highly
extended when cells reenter the cell cycle, and it is perhaps then that the
et al
., 2004; Caligiuri and Beach, 1993; Rib´r
et al.
de novo
production of proteins required for cell cycle progression may be critical.
Another essential point to reemphasize is that the presence or absence
of critical cell cycle proteins within cells is almost always controlled at multiple
levels, transcriptionally and posttranscriptionally. Such systems introduce dou-
ble-check mechanisms, whose success is exemplified by the fact that experimen-
tally manipulating transcription of the gene can have little effect on cell cycle
progression.
VII. RELATED AREAS OF INTEREST AND TOPICS FOR
FURTHER STUDY
Despite much research allowing a reasonably comprehensive picture of cell cycle
regulated gene expression during mitosis in both yeast species, there are still
some large gaps that need to be filled, and much detail to be unraveled. Here, I
highlight some of the major areas and also describe related areas of interest.
A. Histone transcription in budding yeast and G2-M transcription in
fission yeast
Perhaps, the most notable gap in budding yeast is the lack of understanding of
how the periodic expression of the histone genes is controlled, despite these
being the first cell cycle regulated genes to be identified. Histones fall into many
different subtypes, characterized by differing posttranslational modifications,
