Transcriptional regulation of meiosis in budding yeast
Transcriptional regulation of meiosis in budding yeast
Transcriptional regulation of meiosis in budding yeast
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<strong>in</strong>volved with the nuclear cell cycle are required for <strong>meiosis</strong> (Simchen, 1974), and are expressed<br />
<strong>in</strong> both the mitotic and meiotic cycles. The RAD genes are <strong>in</strong>volved with DNA repair as well as <strong>in</strong><br />
checkpo<strong>in</strong>t surveillance, and <strong>in</strong> <strong>meiosis</strong> are required for meiotic recomb<strong>in</strong>ation and surveillance<br />
mechanisms.<br />
Initiation and progression through the meiotic cycle is regulated by a transcriptional cascade<br />
consist<strong>in</strong>g <strong>of</strong> a temporal and programmed expression <strong>of</strong> 6 classes <strong>of</strong> <strong>meiosis</strong>-specific genes (early<br />
I, early II, early middle, middle, mid-late, and late) (Chu et al., 1998; Primig et al., 2000). Fig. 2<br />
shows a schematic draw<strong>in</strong>g <strong>of</strong> this transcriptional cascade, the tim<strong>in</strong>g <strong>of</strong> meiotic events, and the<br />
<strong>in</strong>put <strong>of</strong> the meiotic signals. IME1 is the master regulator gene absolutely required for entry <strong>in</strong>to<br />
the meiotic cycle and the transcription <strong>of</strong> all <strong>meiosis</strong>-specific genes (Kassir et al., 1988; Smith<br />
and Mitchell, 1989). Briefly, <strong>in</strong> the mitotic cell cycle IME1 is silent, as its transcription is<br />
regulated by the three meiotic signals (Kassir et al., 1988). The translation and activity <strong>of</strong> Ime1 is<br />
also regulated by nutrients (Rub<strong>in</strong>-Bejerano et al., 1996; Sherman et al., 1993). IME1 encodes a<br />
transcriptional activator that is directly required for the transcription <strong>of</strong> early <strong>meiosis</strong>-specific<br />
genes (EMG) (Mandel et al., 1994; Smith et al., 1993). The transcription <strong>of</strong> the middle genes<br />
(MMG) depends on the transcription factor, Ndt80, and on the ser<strong>in</strong>e/threon<strong>in</strong>e prote<strong>in</strong> k<strong>in</strong>ase,<br />
Ime2, two early <strong>meiosis</strong>-specific genes whose transcription depends on Ime1 (Chu and<br />
Herskowitz, 1998; Hepworth et al., 1998; Mitchell et al., 1990; Yoshida et al., 1990). In addition,<br />
the function <strong>of</strong> Ime2 is directly regulated by nutrients (Donzeau and Bandlow, 1999; Mitchell et<br />
al., 1990; Yoshida et al., 1990). <strong>Transcriptional</strong> <strong>regulation</strong> <strong>of</strong> mid-late and late genes (LMG) is<br />
less clear, however, their regulated expression depends on the upstream regulators, Ime1, Ime2,<br />
and Ndt80, as well as on nitrogen depletion (Friesen et al., 1997; Kihara et al., 1991).<br />
There is a good correlation between time <strong>of</strong> transcription and meiotic function (Chu et al.,<br />
1998; Primig et al., 2000). The transcription <strong>of</strong> EMG is <strong>in</strong>duced prior to premeiotic DNA<br />
replication, and it <strong>in</strong>cludes genes required for pair<strong>in</strong>g <strong>of</strong> homologous chromosomes (i.e. HOP1),<br />
and meiotic recomb<strong>in</strong>ation (i.e. DMC1). Furthermore, the transcription <strong>of</strong> CDC genes required for<br />
premeiotic DNA replication is <strong>in</strong>duced at this time (i.e. POL1) (Johnston et al., 1986). Middle<br />
genes are <strong>in</strong>duced follow<strong>in</strong>g completion <strong>of</strong> DNA replication and prior to the first nuclear division.<br />
These genes are <strong>in</strong>volved with nuclear division (i.e. CLB1,3,4, CDC26). The late genes are<br />
expressed follow<strong>in</strong>g completion <strong>of</strong> nuclear divisions, and are <strong>in</strong>volved with spore formation and<br />
its maturation (i.e. DIT1, SPS100) (Chu et al., 1998). However, some genes don’t follow this rule.<br />
For example, CLB5,6 are middle genes (Chu et al., 1998) that are required for premeiotic DNA<br />
replication (Dirick et al., 1998; Stuart and Wittenberg, 1998). In the mitotic cell cycle Clb5 is also<br />
required for sp<strong>in</strong>dle orientation, a process occurr<strong>in</strong>g concomitantly with DNA replication (Segal<br />
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