Weickert M J, Adhya S
Laboratory of Molecular Biology, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892.
J Bacteriol. 1993 Jan;175(1):251-8. doi: 10.1128/jb.175.1.251-258.1993.
Two regulatory proteins, Gal repressor and isorepressor, control the expression of the gal and mgl operons in Escherichia coli. The transcription start sites for galR and galS, the genes for the repressor and isorepressor, were determined by primer extension of in vivo transcripts. Study of the promoter-lacZ gene fusions introduced into the chromosome indicated that galS expression was elevated in cells in which the normal galS gene was interrupted, but not in cells in which the galR gene was deleted. When both genes were disrupted, galS expression was further elevated. Expression from the galS promoter was stimulated by the addition of D-fucose, repressed by glucose, and dependent on cyclic AMP receptor protein (CRP). Expression of a similar gene fusion of the galR promoter to lacZ was unregulated. Both galR and galS genes contain two potential operator sites (OE and OI) and a CRP-binding site. The arrangement of OE, OI, and the CRP-binding site in the galS gene is analogous to the arrangement in the gal and mgl promoters, but the arrangement in galR is atypical. The increased concentration of the isorepressor when inducer is present may facilitate early shutoff of the isorepressor-regulated genes of the gal regulon when inducer (substrate) concentration falls.
两种调节蛋白，即半乳糖阻遏蛋白和异阻遏蛋白，控制着大肠杆菌中半乳糖操纵子和mgl操纵子的表达。通过对体内转录本进行引物延伸，确定了阻遏蛋白和异阻遏蛋白的基因galR和galS的转录起始位点。对导入染色体的启动子-lacZ基因融合体的研究表明，在正常galS基因被中断的细胞中galS表达升高，但在galR基因被缺失的细胞中则不然。当两个基因都被破坏时，galS表达进一步升高。galS启动子的表达受到D-岩藻糖的刺激，被葡萄糖抑制，并且依赖于环腺苷酸受体蛋白（CRP）。galR启动子与lacZ的类似基因融合体的表达不受调控。galR和galS基因都包含两个潜在的操纵位点（OE和OI）以及一个CRP结合位点。galS基因中OE、OI和CRP结合位点的排列类似于gal和mgl启动子中的排列，但galR中的排列是非典型的。当诱导剂存在时异阻遏蛋白浓度的增加，可能有助于在诱导剂（底物）浓度下降时，早期关闭半乳糖调节子中异阻遏蛋白调节的基因。
