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Cyclic AMP Receptor Protein-Dependent Repression of Heat-Labile Enterotoxin

机译：不稳定的肠毒素对环AMP受体蛋白的依赖性抑制

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Enterotoxigenic Escherichia coli is a major cause of acute diarrheal illness worldwide and is responsible for high infant and child mortality rates in developing nations. Two types of enterotoxins, one heat labile and the other heat stable, are known to cause diarrhea. The expression of soluble heat-labile toxin is subject to catabolite (glucose) activation, and three binding sites for cAMP receptor protein (CRP or CAP) were identified upstream and within the toxin promoter by DNase I footprinting. One CRP operator is centered at ?31.5, thus encompassing the promoter's ?35 hexamer. Potassium permanganate footprinting revealed that the occupancy of this operator prevents RNA polymerase from forming an open complex in vitro. However, the operator centered at ?31.5 is not sufficient for full repression in vivo because the deletion of the other two CRP binding sites partially relieved the CRP-dependent repression of the heat-labile toxin promoter. In contrast to heat-labile toxin, CRP positively regulates the expression of heat-stable toxin. Thus, the conditions for the optimal expression of one enterotoxin limit the expression of the other. Since glucose inhibits the activity of CRP by suppressing the pathogen's synthesis of cyclic AMP (cAMP), the concentration of glucose in the lumen of the small intestine may determine which enterotoxin is maximally expressed. In addition, our results suggest that the host may also modulate enterotoxin expression because cells intoxicated with heat-labile toxin overproduce and release cAMP.
机译：产肠毒素的大肠埃希菌是全世界急性腹泻病的主要原因，是导致发展中国家婴儿和儿童死亡率高的原因。已知两种肠毒素会引起腹泻，一种是热不稳定的，另一种是热稳定的。可溶性热不稳定毒素的表达受到分解代谢物（葡萄糖）的激活，并且通过DNase I足迹在毒素启动子的上游和内部鉴定了cAMP受体蛋白（CRP或CAP）的三个结合位点。一个CRP操纵子的中心是λ31.5，因此包含启动子的λ35六聚体。高锰酸钾足迹显示该操作员的存在阻止了RNA聚合酶在体外形成开放复合物。但是，以31.5为中心的操纵基因不足以在体内完全抑制，因为其他两个CRP结合位点的缺失部分地缓解了对热不稳定毒素启动子的CRP依赖性抑制。与热不稳定毒素相反，CRP积极调节热稳定毒素的表达。因此，一种肠毒素的最佳表达条件限制了另一种肠毒素的表达。由于葡萄糖通过抑制病原体的环状AMP（cAMP）合成而抑制了CRP的活性，因此小肠腔内葡萄糖的浓度可能决定了哪种肠毒素被最大程度地表达。此外，我们的结果表明，宿主还可以调节肠毒素的表达，因为被热不稳定毒素中毒的细胞会过度产生并释放cAMP。

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