Transcriptional regulation of meiosis in budding yeast

autophosphorylation, including on a conserved tyrosine (Y199) (Rayner et al., 2002).
Phosphorylation of this tyrosine residue in Mck1, similar to other GSK3β homologs, is required
for its kinase activity on exogenous substrates, but not for autophosphorylation (Rayner et al.,
2002). The Ptp2 and Ptp3 tyrosine phosphatases are required for Mck1 dephosphorylation (Zhan
et al., 2000). Deletion of either PTP2 or PTP3 has no effect on meiosis, however, the double
mutant ptp2∆ ptp3∆ is sporulation deficient, it arrests in meiosis prior to premeiotic DNA
replication, with a significant reduction in the expression of IME1 and IME2, and no expression
of middle or late meiosis-specific genes (Zhan et al., 2000). A third tyrosine phosphatase, Yvh1,
might contribute to this regulation (Guan et al., 1992). Deletion of YVH1 leads to minor effects on
the transcription of IME1, a reduced transcription of IME2, and a reduction in the efficiency of
asci formation (Park et al., 1996), but in the double mutant yvh1∆ ptp2∆ sporulation is almost
diminished (Park et al., 1996). Thus, the effect of these tyrosine phosphatases on meiosis may be
partially mediated through Mck1. The transcription of YVH1 is induced upon nitrogen depletion,
suggesting that it might be involved with transmitting the nitrogen signal (Park et al., 1996).
Over expression of Mck1 suppresses the sporulation defect of cells deleted for TPS1 (De Silva-
Udawatta and Cannon, 2001). TPS1 encodes one of the subunits of the trehalose phosphate
synthase complex (Reinders et al., 1997). Thus, Tps1 may either regulate Mck1, or suppression of
mck1∆ by Tps1 is due to unrelated increase in the level of Ime1. The region in IME1 regulated by
Mck1 and its mode of function is not known. However, the additive effects of mutations in
MCK1, MDS3, PMD1, RIM1 and IME4 suggest that these genes affect different regulatory
elements in the IME1 promoter (Su and Mitchell, 1993b).
2. Tup1/Ssn6. The Tup1/Ssn6 complex functions as a general transcriptional repressor upon
recruitment by specific DNA-binding proteins (Keleher et al., 1992). These proteins are also
negative regulators for the transcription of IME1 (Mizuno et al., 1998), but the specific DNAbinding
protein that tethers them to IME1 is not known. Two regions in IME1 promoter respond
to Tup1, the first one is from -914 to – 621 and the second from -1215 to –915 (Mizuno et al.,
1998), functioning as URS in the presence of Tup1, and UAS in the absence of Tup1,
respectively (Mizuno et al., 1998). Since these regions carry the IREd and IREu repeated
elements (Fig. 3) that function as repression and activation sequences, respectively, it should be
interesting to determine if the Tup1/Ssn6 complex mediates its effect through these elements, and
whether Sok2 is the DNA-binding protein that recruits them to these elements.
16