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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However, direct demonstration <strong>of</strong> physical association between Ime1 and Ume6 is still miss<strong>in</strong>g.<br />
The use <strong>of</strong> the two-hybrid assay enables the demonstration that this <strong>in</strong>teraction is negatively<br />
regulated by both glucose and nitrogen. The <strong>in</strong>teraction is absent <strong>in</strong> vegetative growth conditions<br />
with glucose as the sole carbon source, and excessive <strong>in</strong>teraction takes place under meiotic<br />
conditions (SPM media) (Colom<strong>in</strong>a et al., 1999; Rub<strong>in</strong>-Bejerano et al., 1996). Shift<strong>in</strong>g glucose<br />
grown stationary cells to acetate growth media leads to partial <strong>in</strong>teraction (Rub<strong>in</strong>-Bejerano et al.,<br />
1996), suggest<strong>in</strong>g that the <strong>in</strong>teraction <strong>of</strong> Ime1 with Ume6 depends on the absence <strong>of</strong> both glucose<br />
and nitrogen. Furthermore, this <strong>in</strong>teraction is absolutely dependent on Rim11 (Colom<strong>in</strong>a et al.,<br />
1999; Rub<strong>in</strong>-Bejerano et al., 1996), and partially dependent on Rim15 (Vidan and Mitchell,<br />
1997).<br />
Two models were suggested for the role <strong>of</strong> Ime1 Ume6 and Rim11 <strong>in</strong> the transcription <strong>of</strong><br />
EMG. I. Bowdish et al proposed that Ume6 is converted from a repressor to an activator<br />
follow<strong>in</strong>g phosphorylation by Rim11, and that Ime1, which associates with Rim11 (Bowdish et<br />
al., 1994; Rub<strong>in</strong>-Bejerano et al., 1996), is required to recruit Rim11 to Ume6 (Bowdish et al.,<br />
1995). ii. Rub<strong>in</strong>-Bejerano et al proposed that Ume6 recruits Ime1 to the URS1 element present <strong>in</strong><br />
EMG, and that this recruitment promotes the Ime1 transcriptional activation doma<strong>in</strong> to <strong>in</strong>duce the<br />
transcription <strong>of</strong> EMG. Accord<strong>in</strong>g to the later, Rim11 is required for the association between Ime1<br />
and Ume6 (Rub<strong>in</strong>-Bejerano et al., 1996). The follow<strong>in</strong>g results support the second model: i. Ime1<br />
functions as a transcriptional activator (Mandel et al., 1994; Smith et al., 1993). ii. Fusion <strong>of</strong> an<br />
heterologous transcriptional activation doma<strong>in</strong>, that <strong>of</strong> Gal4, to Ime1(id), leads to the expression<br />
<strong>of</strong> the EMG, IME2, and sporulation (Mandel et al., 1994). Moreover, this Ime1(id)-Gal4(ad)<br />
fusion prote<strong>in</strong> promotes <strong>meiosis</strong> <strong>of</strong> ime1∆ diploid cells only when galactose is added to the<br />
sporulation media (Mandel et al., 1994). S<strong>in</strong>ce galactose is required to relieve repression <strong>of</strong> Gal80<br />
on the transcriptional activation <strong>of</strong> Gal4(ad) (Johnston and Carlson, 1992), it was suggested that<br />
the normal function <strong>of</strong> Ime1 is to activate transcription (Mandel et al., 1994). iii. A Gal4(ad)-<br />
Ume6(159-836) fusion prote<strong>in</strong> that is expressed from the IME1 promoter bypasses the<br />
requirement for IME1 for the transcription <strong>of</strong> EMG and sporulation: it leads to expression <strong>of</strong> the<br />
EMG, HOP1, and to 40% sporulation (Rub<strong>in</strong>-Bejerano et al., 1996). iv. A Gal4(ad)-Ume6 fusion<br />
prote<strong>in</strong> can activate the transcription <strong>of</strong> the EMG SPO13 <strong>in</strong> SD media if it carries mutations that<br />
prevent its association with S<strong>in</strong>3 (Washburn and Esposito, 2001). In the absence <strong>of</strong> the Gal4(ad),<br />
these ume6 mutants do not promote expression <strong>of</strong> EMG (Washburn and Esposito, 2001). These<br />
results suggest that by itself Ume6 does not function as a transcriptional activator, and its<br />
conversion <strong>in</strong>to a positive regulator depends on the recruitment <strong>of</strong> the transcriptional activation<br />
doma<strong>in</strong> <strong>of</strong> Ime1 (Rub<strong>in</strong>-Bejerano et al., 1996; Washburn and Esposito, 2001). Currently, this is<br />
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