Johnston M, Flick J S, Pexton T
Department of Genetics, Washington University School of Medicine, St. Louis, Missouri 63110.
Mol Cell Biol. 1994 Jun;14(6):3834-41. doi: 10.1128/mcb.14.6.3834-3841.1994.
Expression of the GAL genes of Saccharomyces cerevisiae is induced during growth on galactose by a well-characterized regulatory mechanism that relieves Gal80p inhibition of the Gal4p transcriptional activator. Growth on glucose overrides induction by galactose. Glucose repression acts at three levels to reduce GAL1 expression: (i) it reduces the level of functional inducer in the cell; (ii) it lowers cellular levels of Gal4p by repressing GAL4 transcription; and (iii) it inhibits Gal4p function through a repression element in the GAL1 promoter. We quantified the amount of repression provided by each mechanism by assaying strains with none, one, two, or all three of the repression mechanisms intact. In a strain lacking all three repression mechanisms, there was almost no glucose repression of GAL1 expression, suggesting that these are the major, possibly the only, mechanisms of glucose repression acting upon the GAL genes. The mechanism of repression that acts to reduce Gal4p levels in the cell is established slowly (hours after glucose addition), probably because Gal4p is stable. By contrast, the repression acting through the upstream repression sequence element in the GAL1 promoter is established rapidly (within minutes of glucose addition). Thus, these three mechanisms of repression collaborate to repress GAL1 expression rapidly and stringently. The Mig1p repressor is responsible for most (possibly all) of these repression mechanisms. We show that for GAL1 expression, mig1 mutations are epistatic to snf1 mutations, indicating that Mig1p acts after the Snf1p protein kinase in the glucose repression pathway, which suggests that Snf1p is an inhibitor of Mig1p.
酿酒酵母的GAL基因在半乳糖上生长时，通过一种特征明确的调控机制被诱导表达，该机制可解除Gal80p对Gal4p转录激活因子的抑制作用。在葡萄糖上生长会抑制半乳糖的诱导作用。葡萄糖阻遏作用在三个水平上降低GAL1的表达：（i）它降低细胞中功能性诱导物的水平；（ii）通过抑制GAL4转录降低细胞中Gal4p的水平；（iii）它通过GAL1启动子中的阻遏元件抑制Gal4p的功能。我们通过检测分别缺失、具有一种、两种或全部三种阻遏机制的菌株，对每种机制所提供的阻遏量进行了量化。在一种缺乏所有三种阻遏机制的菌株中，GAL1的表达几乎没有葡萄糖阻遏现象，这表明这些是作用于GAL基因的葡萄糖阻遏的主要机制，可能也是唯一机制。在细胞中降低Gal4p水平的阻遏机制建立缓慢（添加葡萄糖数小时后），这可能是因为Gal4p很稳定。相比之下，通过GAL1启动子中的上游阻遏序列元件起作用的阻遏作用建立迅速（添加葡萄糖后几分钟内）。因此，这三种阻遏机制协同作用，迅速且严格地抑制GAL1的表达。Mig1p阻遏蛋白负责这些阻遏机制中的大部分（可能是全部）。我们发现，对于GAL1的表达，mig1突变对snf1突变呈上位性，这表明Mig1p在葡萄糖阻遏途径中位于Snf1p蛋白激酶之后起作用，这表明Snf1p是Mig1p的抑制剂。
