英汉对照分子生物学导论 教学课件 ppt 作者 [美]西尔维恩 W.勒潘 王勇 编著Chapter 3.ppt

The best studied operon is the lac operon of E. coli, which contains genes for metabolizing the sugar lactose. It is a good example of how a prokaryote can alter its set of proteins to adapt to nutrient availability in the environment. 研究得最详细的操纵子是大肠杆菌的乳糖操纵子，它含有代谢乳糖的基因。它是原核生物如何调节其多种蛋白质产物以适应环境中营养成分变化的很好的例子。 E. coli generally uses glucose as its energy source. Glucose is relatively simple to metabolize. However, if E. coli is in an environment without glucose, it can shift to use other sources of energy. One possible source is lactose. Lactose is a disaccharide. It is composed of glucose and galactose. Lactose can be broken into glucose and galactose by beta-galactosidase. 大肠杆菌通常使用葡萄糖作为能量来源，葡萄糖比较容易被利用。然而，如果大肠杆菌发现它处于一个没有葡萄糖的环境中时，它也能适应使用其它能量来源。一种可能的来源是乳糖。乳糖是一种双糖，由一个葡萄糖单体和一个半乳糖单体组成。β-半乳糖苷酶能够把乳糖打断产生葡萄糖和半乳糖。 In order to use lactose, E. coli needs to have three extra proteins. First one is lactose permease. This protein brings lactose into the cell. Second one is β-galactosidase. This is an enzyme that breaks lactose into glucose and galactose. The third one is transacetylase. Its function is not clear. 为了代谢乳糖，大肠杆菌需要表达三种额外的蛋白质：第一个是乳糖渗透酶，用于将乳糖送进细胞里；第二个是β-半乳糖苷酶，用于把乳糖分离成葡萄糖和半乳糖；第三个是半乳糖苷转乙酰基酶，其功能尚不清楚。 One important thing is that E. coli should only produce these three proteins only when lactose is present and glucose is not present. Making them under any other condition would be useless. If glucose is present, there is no reason to metabolize lactose, because it is easier to metabolize glucose. If lactose is absent, there is no reason to make these proteins either. 重要的是，这三种蛋白质只有在存在乳糖并且不存在葡萄糖的情况下才应该被产生出来。在任何其它条件下生产它们将是无用的。当存在葡萄糖时，没有理由去代谢乳糖，因为代谢葡萄糖更容易。当乳糖不存在时，就更没有理由产生这些用于代谢乳糖的蛋白质了。 How can E. coli ensure that these three proteins are only produced in the appropriate condition? In fact, these three genes are organized into an operon. And this operon is subject to both negative and positive regulation. The Negative Regulation is realized through using the lac Repre