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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egulator <strong>of</strong> Cyr1, CDC25 encodes the Ras GDP/GTP exchange factor, which serves as a positive<br />
regulator <strong>of</strong> adenylate cyclase (Broach, 1991; Broek et al., 1987; Toda et al., 1985)]. On the other<br />
hand, mutations that cause constitutive PKA activity, such as RAS2-val19 (activated Ras) and<br />
bcy1 [the regulatory subunit <strong>of</strong> PKA (Broach, 1991)] are sporulation deficient, and are suppressed<br />
by over-expression <strong>of</strong> IME1 (Matsuura et al., 1990). The repression activity <strong>of</strong> UCS1 is reduced<br />
<strong>in</strong> vegetatively grown cdc25-5 cells <strong>in</strong>cubated at the non-permissive temperature (Fig. 5). Upon<br />
nitrogen depletion (SPM), this phenomenon is not observed; the same levels <strong>of</strong> expression are<br />
observed <strong>in</strong> cells <strong>in</strong>cubated at the permissive or restrictive temperature (Fig. 5). These results<br />
suggest that Cdc25 transmits the nitrogen signal to UCS1. Cdc25 is a positive regulator <strong>of</strong> both<br />
the, cAMP/PKA (Broek et al., 1987) and MAPK (Shenhar, 2001) signal transduction pathways.<br />
Therefore, further experiments are required to determ<strong>in</strong>e if this effect <strong>of</strong> Cdc25 is mediated<br />
through PKA, MAPK or a different signal pathway.<br />
MDS3 and its homologue, PMD1, are negative regulators <strong>of</strong> IME1 transcription <strong>in</strong> vegetative<br />
growth media with acetate as the sole carbon source. Deletion <strong>of</strong> these genes results <strong>in</strong> an<br />
<strong>in</strong>crease <strong>in</strong> the transcription <strong>of</strong> IME1 and the EMG IME2 and HOP1 (Benni and Neigeborn,<br />
1997). In addition, when stationary phase mds3∆ pmd1∆ diploid cells are shifted from SD to SA<br />
media, growth is ceased and about 10% <strong>of</strong> the cells enter and complete the meiotic cycle. These<br />
results suggest that Mds3/Pmd1 function upstream <strong>of</strong> Ime1, <strong>in</strong> prevent<strong>in</strong>g cell cycle arrest <strong>in</strong> the<br />
presence <strong>of</strong> nitrogen (Benni and Neigeborn, 1997). Growth arrest and sporulation are suppressed<br />
<strong>in</strong> cells carry<strong>in</strong>g the RAS2-val19 mutation, suggest<strong>in</strong>g that these genes might be upstream<br />
activators <strong>of</strong> the Ras pathway (Benni and Neigeborn, 1997). The region <strong>in</strong> IME1 that is regulated<br />
by these genes is not known. It will be <strong>in</strong>terest<strong>in</strong>g to determ<strong>in</strong>e if their effect is mediated through<br />
the UCS1 element.<br />
As discussed above, the Mata1 and Matα2 gene products are required to <strong>in</strong>duce expression <strong>in</strong><br />
the absence <strong>of</strong> nitrogen, suggest<strong>in</strong>g that UCS1 might also mediate the MAT signal. However, the<br />
repression activity <strong>of</strong> UCS1 and its relief (<strong>in</strong> a UASHIS4-UCS1-his4-lacZ reporter) is identical <strong>in</strong><br />
isogenic haploid and diploid cells (Boger-Nadjar, 2000), <strong>in</strong>dicat<strong>in</strong>g that the activity <strong>of</strong> UCS1 is<br />
not regulated by MAT. However, it is still possible that with<strong>in</strong> the context <strong>of</strong> IME1 promoter,<br />
UCS1 might <strong>in</strong>teract with the UCS3 and/or UCS4 elements that mediate MAT <strong>regulation</strong>.<br />
C. The glucose Signal<br />
The UAS activity <strong>of</strong> IME1 is conf<strong>in</strong>ed to UCS2, a region that conta<strong>in</strong>s alternate positive and<br />
negative elements [Fig. 3, (Sagee et al., 1998)]. Nested deletions <strong>of</strong> either one <strong>of</strong> its three positive<br />
elements, UASru, IREu, and UASrm lead to a reduction <strong>in</strong> expression <strong>of</strong> ime1-lacZ <strong>in</strong> SPM (Fig.<br />
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