📚 Gene Control in Prokaryotes | 原核生物的基因调控
Prokaryotes such as Escherichia coli live in rapidly changing environments. A bacterium must switch genes on or off to avoid wasting energy and molecules. Gene control in prokaryotes is mainly transcriptional and is organised through operons, allowing coordinate regulation of genes with related functions.
大肠杆菌等原核生物生活在快速变化的环境中。细菌必须开启或关闭基因，以避免浪费能量和分子。原核生物的基因调控主要发生在转录水平，并通过操纵子组织起来，使功能相关的基因能够协调调控。
1. Why Prokaryotes Regulate Genes | 原核生物为何要调控基因
Prokaryotes do not have a nucleus, so transcription and translation are coupled in the cytoplasm. This allows very rapid responses to environmental change. However, making enzymes is costly; if a nutrient is absent, synthesising its catabolic enzymes wastes amino acids and ATP. Therefore bacteria often repress unnecessary genes and activate only those needed for available substrates.
原核生物没有细胞核，因此转录和翻译在细胞质中偶联进行。这使得细菌能对环境变化作出非常快速的反应。然而，合成酶的成本很高；如果某种营养物质不存在，合成其分解代谢酶就会浪费氨基酸和ATP。因此细菌通常抑制不必要的基因，只激活当前底物所需的基因。
2. Operon Structure: Promoter, Operator, Structural Genes | 操纵子结构：启动子、操纵基因、结构基因
An operon is a cluster of structural genes under the control of a single promoter. Its key parts are: promoter (P) where RNA polymerase binds; operator (O), a switch region that can be blocked by a repressor; and structural genes coding for enzymes. A separate regulatory gene encodes a repressor or activator protein.
操纵子是由单一启动子控制的一组结构基因。其关键部分是：启动子（P），RNA聚合酶在此结合；操纵基因（O），一个可被阻遏蛋白阻断的开关区域；以及编码酶的结构基因。另一个调节基因编码阻遏蛋白或激活蛋白。
3. The lac Operon: Negative Inducible Control | lac操纵子：负向诱导调控
In E. coli, the lac operon contains lacZ, lacY and lacA. The regulatory gene lacI encodes the lac repressor. In the absence of lactose, the repressor binds to the operator. This prevents RNA polymerase from moving through the structural genes, so transcription is switched off.
在大肠杆菌中，lac操纵子包含lacZ、lacY和lacA。调节基因lacI编码lac阻遏蛋白。在没有乳糖时，阻遏蛋白与操纵基因结合。这阻止RNA聚合酶通过结构基因，因此转录被关闭。
4. Role of Lactose and Allolactose | 乳糖与别乳糖的作用
When lactose enters the cell, β-galactosidase converts a small amount into allolactose. Allolactose acts as the inducer. It binds to the lac repressor, changing its shape so that the repressor can no longer bind the operator. RNA polymerase can then transcribe the structural genes. This is called negative inducible control because the default repressor is removed by an inducer.
当乳糖进入细胞后，β-半乳糖苷酶将少量乳糖转化为别乳糖。别乳糖作为诱导物。它与lac阻遏蛋白结合，改变其形状，使阻遏蛋白不能再与操纵基因结合。随后RNA聚合酶可以转录结构基因。这称为负向诱导调控，因为默认存在的阻遏蛋白被诱导物去除。
5. CAP and Positive Control | CAP与正调控
If glucose is also present, bacteria prefer glucose, so the lac operon remains low. When glucose levels fall, cAMP concentration rises. cAMP binds to catabolite activator protein (CAP). The cAMP-CAP complex binds near the lac promoter and helps RNA polymerase attach. This positive control ensures lactose enzymes are made efficiently only when glucose is scarce and lactose is available.
如果同时存在葡萄糖，细菌优先利用葡萄糖，因此lac操纵子仍保持低水平表达。当葡萄糖水平下降时，cAMP浓度升高。cAMP与分解代谢物激活蛋白（CAP）结合。cAMP-CAP复合物结合在lac启动子附近，帮助RNA聚合酶结合。这种正调控确保只有在葡萄糖稀少且乳糖可用时，才高效合成乳糖代谢酶。
6. The trp Operon: Repressible Control | trp操纵子：可阻遏调控
The trp operon contains five structural genes (trpE, trpD, trpC, trpB, trpA) encoding enzymes for tryptophan synthesis. Its regulatory gene trpR produces the trp repressor protein, but this aporepressor is inactive on its own. The operon is transcribed when tryptophan levels are low.
trp操纵子含有五个结构基因（trpE、trpD、trpC、trpB、trpA），编码合成色氨酸的酶。其调节基因trpR产生trp阻遏蛋白，但这种脱辅基阻遏蛋白本身没有活性。当色氨酸水平低时，该操纵子进行转录。
7. Co-repressor and Negative Feedback | 共阻遏物与负反馈
Tryptophan itself acts as a co-repressor. When tryptophan is abundant, it binds to the trp repressor, activating it. The repressor-co-repressor complex binds to the operator and blocks transcription. As tryptophan is consumed and its concentration falls, the co-repressor detaches, the repressor becomes inactive, and transcription resumes. This is negative repressible control and a form of end-product feedback inhibition at gene level.
色氨酸本身作为共阻遏物。当色氨酸充足时，它与trp阻遏蛋白结合并使其活化。阻遏蛋白-共阻遏物复合物与操纵基因结合，阻止转录。当色氨酸被消耗、浓度下降时，共阻遏物脱离，阻遏蛋白失活，转录重新开始。这属于负向阻遏调控，是终产物在基因水平上的反馈抑制。
8. Coupled Transcription and Translation | 转录与翻译偶联
Because prokaryotes lack a nuclear envelope, ribosomes can attach to mRNA while the mRNA is still being synthesised. This coupling allows a bacterium to begin making enzymes within minutes of gene activation. It also means regulation does not rely on RNA processing or nuclear export, unlike in eukaryotes.
由于原核生物没有核膜，核糖体可以在mRNA仍在合成时就与之结合。这种偶联使细菌在基因激活后几分钟内就能开始制造酶。这也意味着调控不依赖于RNA加工或核输出，与真核生物不同。
9. Polycistronic mRNA | 多顺反子mRNA
The mRNA produced from an operon is polycistronic, meaning one mRNA carries several coding regions for different polypeptides. Ribosomes can translate each cistron separately, producing all the enzymes needed in a pathway from a single transcript. This coordinate expression is efficient for metabolic pathways.
由操纵子转录产生的mRNA是多顺反子，即一条mRNA携带多个不同多肽的编码区。核糖体可以分别翻译每个顺反子，从一条转录本产生代谢途径所需的全部酶。这种协调表达对代谢途径非常高效。
10. Comparing Inducible and Repressible Systems | 诱导型与阻遏型系统比较
The lac operon is an inducible system: it is off until turned on by lactose/allolactose. The trp operon is a repressible system: it is on until turned off by tryptophan. Both use a repressor-operator interaction, but the effector molecule has opposite effects on the repressor.
lac操纵子是诱导型系统：在乳糖/别乳糖作用下才开启。trp操纵子是阻遏型系统：在色氨酸作用下才关闭。两者都依赖阻遏蛋白与操纵基因的相互作用，但效应分子对阻遏蛋白的作用相反。
| Feature | lac operon | trp operon |
|---|---|---|
| Type | Inducible | Repressible |
| Effector | Allolactose (inducer) | Tryptophan (co-repressor) |
| Default state | Repressed | Active |
| Effect of effector on repressor | Inactivates repressor | Activates repressor |
11. Experimental Evidence: Jacob and Monod | 实验证据：Jacob和Monod
The operon model was proposed by François Jacob and Jacques Monod in 1961 using E. coli lactose metabolism mutants. Mutations in lacI caused constitutive expression, suggesting a repressor gene. Mutations in the operator prevented repressor binding but did not affect the repressor itself. These genetic analyses distinguished regulatory sites from structural genes.
1961年，François Jacob和Jacques Monod利用大肠杆菌乳糖代谢突变体提出了操纵子模型。lacI突变导致组成型表达，提示存在阻遏基因。操纵基因突变阻止阻遏蛋白结合，但不影响阻遏蛋白本身。这些遗传分析区分了调控位点与结构基因。
12. Exam Tips and Common Misconceptions | 考试技巧与常见误区
Students often confuse the operator with the promoter. The promoter is the RNA polymerase binding site; the operator is the repressor binding site. Another common error is stating that lactose directly binds the operator. It is allolactose that binds the repressor. Also remember that glucose regulates the lac operon through cAMP-CAP, not by changing lactose binding.
学生常混淆操纵基因与启动子。启动子是RNA聚合酶结合位点；操纵基因是阻遏蛋白结合位点。另一个常见错误是认为乳糖直接与操纵基因结合。实际上与阻遏蛋白结合的是别乳糖。还要记住葡萄糖通过cAMP-CAP调节lac操纵子，而不是改变乳糖的结合。
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