Inada T, Kimata K, Aiba H
Department of Molecular Biology, School of Science, Nagoya University, Chikusa, Japan.
Genes Cells. 1996 Mar;1(3):293-301. doi: 10.1046/j.1365-2443.1996.24025.x.
The inhibition of beta-galactosidase expression in glucose-lactose diauxie is a typical example of the glucose effect in Escherichia coli. It is generally believed that glucose exerts its effect at least partly by reducing the intracellular cAMP level. However, there is no direct evidence that the inhibitory effect of glucose on the expression of the lac operon is mediated by a reduction of the cAMP level in the glucose-lactose system.
To examine the roles of cAMP and the cAMP receptor protein (CRP) in the glucose effect, the intracellular levels of these factors were determined during diauxic growth in a glucose-lactose medium. We found that the levels of cAMP and CRP in a lactose-grown phase were not higher than those in a glucose-grown phase, although the cAMP levels increased transiently during the lag phase. The addition of exogenous cAMP eliminated diauxic growth but did not eliminate glucose repression. Glucose repression and diauxie were observed in cells which lack cAMP but produce a cAMP-independent CRP. In addition, inactivation of the lac repressor by the disruption of the lacI gene or the addition of IPTG, eliminated glucose repression.
We conclude that the repression of beta-galactosidase expression by glucose is not due to the reduction of the cAMP-CRP level but due to an inducer exclusion mechanism which is mediated by the phosphoenolpyruvate-dependent sugar phosphotransferase system.
在葡萄糖-乳糖双相生长中β-半乳糖苷酶表达的抑制是大肠杆菌中葡萄糖效应的典型例子。一般认为葡萄糖至少部分地通过降低细胞内cAMP水平发挥其作用。然而，没有直接证据表明在葡萄糖-乳糖系统中葡萄糖对乳糖操纵子表达的抑制作用是由cAMP水平降低介导的。
为了研究cAMP和cAMP受体蛋白（CRP）在葡萄糖效应中的作用，在葡萄糖-乳糖培养基双相生长期间测定了这些因子的细胞内水平。我们发现，尽管在迟缓期cAMP水平短暂升高，但在乳糖生长阶段cAMP和CRP的水平并不高于葡萄糖生长阶段。添加外源cAMP消除了双相生长，但没有消除葡萄糖阻遏。在缺乏cAMP但产生不依赖cAMP的CRP的细胞中观察到葡萄糖阻遏和双相生长。此外，通过破坏lacI基因或添加IPTG使lac阻遏物失活，消除了葡萄糖阻遏。
我们得出结论，葡萄糖对β-半乳糖苷酶表达的阻遏不是由于cAMP-CRP水平的降低，而是由于磷酸烯醇丙酮酸依赖性糖磷酸转移酶系统介导的诱导物排除机制。
