Transcriptional regulation of meiosis in budding yeast

I. Introduction
The budding yeast Saccharomyces cerevisiae is a simple unicellular eukaryote exhibiting several
optional developmental pathways. Fig. 1 schematically illustrates the developmental options of
MATa/MATα diploid cells. In the presence of both carbon and nitrogen sources both haploid and
diploid cells adopt the yeast form morphology; upon nitrogen limitation, and in the presence of
high levels of glucose, a dimorphic transition to a filamentous growth takes places [For recent
reviews see (Gancedo, 2001; Lengeler et al., 2000; Pan et al., 2000)]. Haploid as well as diploid
cells manifesting these forms propagate by the mitotic cell cycle. Upon nitrogen depletion the
developmental decision made by the cells depends on the presence or absence of a fermentable
carbon source such as glucose, as well as on the information at the MAT locus. In the presence of
functional MATa1 and MATα2 alleles, regardless of ploidy, i.e. MATa/MATα diploids,
MATa/MATα disomic cells, and haploid cells expressing both MAT alleles, cells enter the meiotic
cycle (Kassir and Simchen, 1976; Kassir and Simchen, 1985; Rine and Herskowitz, 1987; Roman
and Sands, 1953; Roth and Fogel, 1971). In the presence of only a single functional allele, i.e.
MATa and MATα haploids or MATa/MATa and MATα/MATα diploids, depletion of nitrogen
leads to a cell cycle arrest at G1 (Kassir and Simchen, 1976; Roman and Sands, 1953; Strathern et
al., 1981). The last developmental option S. cerevisiae cells possess is that of mating, cells
carrying only one of the two MAT alleles can respond to the pheromone secreted by cells
expressing the other MAT allele by temporal arrest in G1, mate, and then resume cell growth
[reviews on the mating process as well as on how the MAT alleles control cell type are (Fields,
1990; Herskowitz, 1995; Sprague, 1991)].
In this review we focus on how the meiotic signals, namely, the presence of the MATa and
MATα alleles, the presence of a non-fermentable carbon source, the absence of glucose and
nitrogen, control initiation and progression through the meiotic cycle. We focus on transcriptional
regulation rather than the function of the proteins involved with the specific meiotic events [for
reviews on these aspects of meiosis see (Kupiec et al., 1997; Roeder, 1997)].
II. Transcriptional cascade governs initiation of meiosis.
Entry and progression through the meiotic cycle depends on the expression and activity of many
genes that can be roughly divided into 3 groups: meiosis-specific, cell-division cycle (CDC) and
radiation sensitive (RAD) genes. The meiosis-specific genes are expressed only under meiotic
conditions, and their function is required only in meiosis. The cell-division cycle genes that are
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