Parker L L, Hall B G
Department of Molecular and Cell Biology, University of Connecticut, Storrs 06269.
Genetics. 1990 Mar;124(3):455-71. doi: 10.1093/genetics/124.3.455.
Wild-type Escherichia coli are not able to utilize beta-glucoside sugars because the genes for utilization of these sugars are cryptic. Spontaneous mutations in the cel operon allow its expression and enable the organism to ferment cellobiose, arbutin and salicin. In this report we describe the structure and nucleotide sequence of the cel operon. The cel operon consists of five genes: celA, whose function is unknown; celB and celC which encode phosphoenolpyruvate-dependent phosphotransferase system enzyme IIcel and enzyme IIIcel, respectively, for the transport and phosphorylation of beta-glucoside sugars; celD, which encodes a negative regulatory protein; and celF, which encodes a phospho-beta-glucosidase that acts on phosphorylated cellobiose, arbutin and salicin. The mutationally activated cel operon is induced in the presence of its substrates, and is repressed in their absence. A comparison of proteins encoded by the cel operon with functionally equivalent proteins of the bgl operon, another cryptic E. coli gene system responsible for the catabolism of beta-glucoside sugars, revealed no significant homology between these two systems despite common functional characteristics. The celD and celF encoded repressor and phospho-beta-glucosidase proteins are homologous to the melibiose regulatory protein and to the melA encoded alpha-galactosidase of E. coli, respectively. Furthermore, the celC encoded PEP-dependent phosphotransferase system enzyme IIIcel is strikingly homologous to an enzyme IIIlac of the Gram-positive organism Staphylococcus aureus. We conclude that the genes for these two enzyme IIIs diverged much more recently than did their hosts, indicating that E. coli and S. aureus have undergone relatively recent exchange of chromosomal genes.
野生型大肠杆菌无法利用β-葡萄糖苷糖，因为利用这些糖的基因是隐性的。cel操纵子中的自发突变使其得以表达，并使该生物体能够发酵纤维二糖、熊果苷和水杨苷。在本报告中，我们描述了cel操纵子的结构和核苷酸序列。cel操纵子由五个基因组成：celA，其功能未知；celB和celC，分别编码磷酸烯醇丙酮酸依赖性磷酸转移酶系统的酶IIcel和酶IIIcel，用于β-葡萄糖苷糖的转运和磷酸化；celD，编码一种负调控蛋白；celF，编码一种磷酸-β-葡萄糖苷酶，作用于磷酸化的纤维二糖、熊果苷和水杨苷。突变激活的cel操纵子在其底物存在时被诱导，在底物不存在时被抑制。将cel操纵子编码的蛋白质与bgl操纵子（另一个负责β-葡萄糖苷糖分解代谢的隐性大肠杆菌基因系统）功能等效的蛋白质进行比较，结果显示尽管这两个系统具有共同的功能特征，但它们之间没有显著的同源性。celD和celF编码的阻遏蛋白和磷酸-β-葡萄糖苷酶蛋白分别与大肠杆菌的蜜二糖调节蛋白和melA编码的α-半乳糖苷酶同源。此外，celC编码的依赖磷酸烯醇丙酮酸的磷酸转移酶系统酶IIIcel与革兰氏阳性菌金黄色葡萄球菌的酶IIIlac惊人地同源。我们得出结论，这两种酶III的基因分歧比它们的宿主分歧要近得多，这表明大肠杆菌和金黄色葡萄球菌最近经历了染色体基因的交换。
