Mahadevan S, Reynolds A E, Wright A
J Bacteriol. 1987 Jun;169(6):2570-8. doi: 10.1128/jb.169.6.2570-2578.1987.
We have analyzed the functions encoded by the bgl operon in Escherichia coli K-12. Based on the ability of cloned regions of the operon to complement a series of Bgl- point mutations, we show that the three bgl structural genes, bglC, bglS, and bglB, are located downstream of the regulatory locus bglR in the order indicated. Using a bgl-lacZ transcriptional fusion, we show that bglC and bglS are involved in regulating operon expression. The presence of the bglC gene in trans is absolutely required for the expression of the fusion, which is constitutive when only the bglC gene is present. When the bglC and the bglS genes are both present in the cell, expression of the fusion requires a beta-glucoside inducer. From these observations, we conclude that (i) the bglC gene encodes a positive regulatory of bgl operon expression and (ii) the bglS gene encodes a negative regulator of operon expression, causing the requirement for a beta-glucoside inducer. These conclusions are supported by our observations that (i) a majority of bglC mutants exhibits a Bgl- phenotype, whereas rare trans-dominant mutations in bglC result in constitutive expression of the bgl operon and the fusion, and (ii) mutations in the bglS gene lead to constitutive expression of the fusion. Based on several lines of evidence presented, we propose that the bglS gene product has an additional role as a component of the beta-glucoside transport system.
我们分析了大肠杆菌K-12中bgl操纵子编码的功能。基于操纵子克隆区域对一系列Bgl-点突变的互补能力，我们表明，三个bgl结构基因bglC、bglS和bglB，按所示顺序位于调控位点bglR的下游。使用bgl-lacZ转录融合，我们表明bglC和bglS参与调控操纵子表达。反式存在bglC基因对于融合体的表达是绝对必需的，当仅存在bglC基因时，融合体的表达是组成型的。当细胞中同时存在bglC和bglS基因时，融合体的表达需要β-葡萄糖苷诱导剂。从这些观察结果中，我们得出结论：（i）bglC基因编码bgl操纵子表达的正调控因子；（ii）bglS基因编码操纵子表达的负调控因子，导致需要β-葡萄糖苷诱导剂。我们的以下观察结果支持了这些结论：（i）大多数bglC突变体表现出Bgl-表型，而bglC中罕见的反式显性突变导致bgl操纵子和融合体的组成型表达；（ii）bglS基因中的突变导致融合体的组成型表达。基于所提供的几条证据，我们提出bglS基因产物作为β-葡萄糖苷转运系统的一个组成部分具有额外的作用。
