Three enzymes of lac Operon Transcription lacZ lacY lacA break the lactose down into its two β-Galactosidase Permease Transacetylase component sugars:galactose and glucose lacZ lacY lacA -igure 7.2 CHOH CH OH CH.OH Transport the OH lactose into the cells B-galactosidase Lactose Galactose Glucose Function unknown Polycistonic message:starting from a single Cistron means“gene” promoter.Thus,they can all be controlled together simply by controlling that promoter
lacZ lacY Transport the lactose into the cells Function unknown lacA Polycistonic message: starting from a single promoter. Thus, they can all be controlled together simply by controlling that promoter Cistron means “gene” Three enzymes of lac Operon break the lactose down into its two component sugars: galactose and glucose lacZ lacY lacA
A regulator binds a target site on DNA operon control Model Regulator protein Cvirtuated wwwergito Operator Regulatory Control (操纵基因) gene sites Struc mRNA Z Lactose ol ▣ Two types of control Regulator gene Target site Structural gene negative control (negative transcription regulation) positive control (positive transcription regulation) Negative ·I gene activator(apoinducer) Positive 无辅基诱导物
operon control Model Operator (操纵基因) Two types of control • I gene repressor Negative activator (apoinducer) Positive 无辅基诱导物 ✓negative control (negative transcription regulation) ✓positive control（positive transcription regulation）
Negative control Repressor binding on O site hold back transcription negative control is like the brake of a car: You need to release the brake for the car to move. √The“brake'”in negtive control is a protein called the lac repressor(阻遏物)，which keeps the operon turned off (or repressed)as long as lactose is absent
✓negative control is like the brake of a car: You need to release the brake for the car to move. ✓ The “brake” in negtive control is a protein called the lac repressor(阻遏物), which keeps the operon turned off (or repressed) as long as lactose is absent. Repressor binding on O site hold back transcription Negative control
Positive control activator binding front p site Activate transcription positive control is like the accelerator pedal.In the case of the lac operon,removing the repressor from the operator (releasing the brake)is not enough to activate the operon.An additional positive factor called an activator( 化物激活子)is needed
positive control is like the accelerator pedal. In the case of the lac operon, removing the repressor from the operator (releasing the brake) is not enough to activate the operon. An additional positive factor called an activator(活 化物/激活子) is needed activator binding front p site Activate transcription Positive control
小结 √乳糖在大肠杆菌中的新陈代谢是在两种酶的作用下进 行的，可能与第三种酶有关。 √编码这三种酶的基因成簇地排列在一起，它们从同一 个启动子处开始，转录成一个多顺反子。这三种基因 在功能上相关，所以互相连接在一起表达，一起关闭 或开启。 √负调控保证了乳糖操纵子在乳糖不存在时处于关闭状 态，而正调控保证了乳糖操纵子在葡萄糖存在时处于 关闭状态，甚至在乳糖和葡萄糖一起存在时也处于关 闭状态
小结 ✓ 乳糖在大肠杆菌中的新陈代谢是在两种酶的作用下进 行的，可能与第三种酶有关。 ✓ 编码这三种酶的基因成簇地排列在一起，它们从同一 个启动子处开始，转录成一个多顺反子。这三种基因 在功能上相关，所以互相连接在一起表达，一起关闭 或开启。 ✓ 负调控保证了乳糖操纵子在乳糖不存在时处于关闭状 态，而正调控保证了乳糖操纵子在葡萄糖存在时处于 关闭状态，甚至在乳糖和葡萄糖一起存在时也处于关 闭状态