📚 Molecular Mechanisms of Gene Regulation in Prokaryotes | 原核生物基因调控的分子机制
Gene regulation in prokaryotes is a fundamental topic in CIE A-Level Biology, focusing on how bacteria control gene expression in response to environmental changes. Since prokaryotes lack a nucleus, their regulatory mechanisms operate directly at the level of transcription, primarily through the action of regulatory proteins that bind to specific DNA sequences. This article provides a comprehensive yet exam-focused review of the molecular mechanisms underlying prokaryotic gene regulation, with special emphasis on the lac operon and trp operon.
原核生物的基因调控是 CIE A-Level 生物学的核心考点，重点探讨细菌如何根据环境变化控制基因表达。由于原核生物没有细胞核，其调控机制直接作用于转录水平，主要通过调控蛋白与特定 DNA 序列的结合来实现。本文将从考试角度出发，系统梳理原核生物基因调控的分子机制，重点分析 lac 操纵子和 trp 操纵子。
1. Why Do Prokaryotes Need Gene Regulation? | 原核生物为何需要基因调控？
Prokaryotes live in constantly changing environments. It would be wasteful for a bacterial cell to synthesise all possible enzymes at all times, regardless of whether the corresponding substrates are present. By regulating gene expression, bacteria can produce enzymes only when they are needed, thereby conserving energy and resources. This metabolic efficiency gives bacteria a significant survival advantage.
原核生物生活在不断变化的环境中。如果细菌无论底物是否存在，都持续合成所有可能的酶，这将造成极大的资源浪费。通过调控基因表达，细菌仅在需要时才合成相应的酶，从而节约能量和物质，这种代谢效率赋予细菌显著的生存优势。
For example, E. coli can use glucose as its preferred carbon source. When glucose is unavailable but lactose is present, the bacterium must produce the enzymes needed to break down lactose. This switch between carbon sources is achieved through precise gene regulatory mechanisms.
例如，大肠杆菌优先利用葡萄糖作为碳源。当葡萄糖缺乏而乳糖存在时，细菌必须合成分解乳糖所需的酶。这种碳源之间的切换正是通过精确的基因调控机制实现的。
2. Key Regulatory Elements in Prokaryotic DNA | 原核生物 DNA 中的关键调控元件
Before examining the operon model, it is essential to understand the DNA elements involved in prokaryotic gene regulation. These elements are specific sequences on the DNA that interact with RNA polymerase and regulatory proteins.
在研究操纵子模型之前，必须先了解参与原核生物基因调控的 DNA 元件。这些元件是 DNA 上的特定序列，它们与 RNA 聚合酶和调控蛋白相互作用。
- Promoter: The DNA sequence where RNA polymerase binds to initiate transcription.
- Operator: A short DNA sequence between the promoter and the structural genes, where a repressor protein can bind to block transcription.
- Structural genes: Genes that code for enzymes or other proteins involved in a metabolic pathway.
- Regulatory gene: A gene located outside the operon that codes for a regulatory protein, such as a repressor or activator.
- 启动子：RNA 聚合酶结合并启动转录的 DNA 序列。
- 操纵基因：位于启动子和结构基因之间的一段短 DNA 序列，阻遏蛋白可与之结合以阻断转录。
- 结构基因：编码代谢途径中相关酶或其他蛋白质的基因。
- 调控基因：位于操纵子之外、编码调控蛋白（如阻遏蛋白或激活蛋白）的基因。
Prokaryotic operon structure: Promoter → Operator → Structural Genes
原核操纵子结构：启动子 → 操纵基因 → 结构基因
3. The Operon Model: An Overview | 操纵子模型概述
The operon is a unit of coordinated gene expression found in prokaryotes. It consists of a cluster of structural genes transcribed together as a single mRNA molecule, along with the promoter and operator sequences that control their transcription. This arrangement allows bacteria to regulate a group of related genes simultaneously.
操纵子是原核生物中基因协调表达的单位。它由一个结构基因簇（共同转录为一条 mRNA）以及控制其转录的启动子和操纵基因序列组成。这种结构使细菌能够同时调控一组相关基因的表达。
The operon model was first proposed by François Jacob and Jacques Monod in 1961, based on their studies of lactose metabolism in E. coli. Their work laid the foundation for our understanding of gene regulation and earned them the Nobel Prize in 1965.
操纵子模型由 François Jacob 和 Jacques Monod 于 1961 年基于对大肠杆菌乳糖代谢的研究首次提出。他们的工作为基因调控的理解奠定了基础，并为他们赢得了 1965 年的诺贝尔奖。
There are two main types of operons: inducible operons, such as the lac operon, which are usually off but can be turned on; and repressible operons, such as the trp operon, which are usually on but can be turned off.
操纵子主要分为两类：诱导型操纵子（如 lac 操纵子），通常关闭但可被开启；阻遏型操纵子（如 trp 操纵子），通常开启但可被关闭。
4. The lac Operon: Structure and Components | lac 操纵子：结构与组成
The lac operon of E. coli is the classic example of an inducible operon. It is responsible for the uptake and metabolism of lactose. The operon consists of three structural genes:
大肠杆菌的 lac 操纵子是诱导型操纵子的经典范例，负责乳糖的吸收和代谢。该操纵子包含三个结构基因：
- lacZ: Codes for β-galactosidase, which hydrolyses lactose into glucose and galactose.
- lacY: Codes for permease, which transports lactose into the cell.
- lacA: Codes for transacetylase, whose exact function is not essential for lactose metabolism but is involved in detoxification.
- lacZ：编码 β-半乳糖苷酶，将乳糖水解为葡萄糖和半乳糖。
- lacY：编码通透酶，负责将乳糖转运进入细胞。
- lacA：编码转乙酰酶，其确切功能对乳糖代谢并非必需，但参与解毒过程。
Upstream of these structural genes are the promoter and operator sequences. The regulatory gene lacI is located upstream of the operon and codes for the lac repressor protein. Importantly, the lac repressor is synthesised constitutively, meaning it is always produced at a low level.
在这些结构基因的上游是启动子和操纵基因序列。调控基因 lacI 位于操纵子上游，编码 lac 阻遏蛋白。值得注意的是，lac 阻遏蛋白是组成型合成的，即始终以较低水平持续合成。
| Component | 组件 | Function | 功能 |
| Promoter (P) | RNA polymerase binding site | RNA 聚合酶结合位点 |
| Operator (O) | Repressor binding site | 阻遏蛋白结合位点 |
| lacZ | β-galactosidase | β-半乳糖苷酶 |
| lacY | Permease | 通透酶 |
| lacA | Transacetylase | 转乙酰酶 |
| lacI | Codes for repressor protein | 编码阻遏蛋白 |
5. The lac Operon: When Lactose Is Absent | lac 操纵子：乳糖不存在时
When lactose is absent from the environment, the lac operon is switched off. The lac repressor protein, produced by the regulatory gene lacI, binds specifically to the operator sequence. This binding physically obstructs RNA polymerase, preventing it from moving from the promoter to the structural genes. As a result, transcription of lacZ, lacY, and lacA does not occur, and no lactose-metabolising enzymes are produced.
当环境中不存在乳糖时，lac 操纵子处于关闭状态。由调控基因 lacI 产生的 lac 阻遏蛋白特异性结合到操纵基因序列上。这种结合在空间上阻碍了 RNA 聚合酶，使其无法从启动子移动到结构基因。因此，lacZ、lacY 和 lacA 的转录不会发生，乳糖代谢酶也不会被合成。
Lactose absent → Repressor binds operator → Transcription blocked
乳糖不存在 → 阻遏蛋白结合操纵基因 → 转录被阻断
A key exam point is that this system is not absolutely perfect. A few molecules of mRNA may occasionally be transcribed, but the level of enzyme production is extremely low. This is referred to as basal (or leaky) expression. The small amount of permease produced under these conditions is actually important because it allows the cell to detect and import lactose when it first appears in the environment.
一个关键考点是：该系统并非绝对严密。偶尔可能会有少量 mRNA 被转录，但酶产量极低，这被称为基础（或渗漏）表达。在这种条件下产生少量通透酶实际上非常重要，因为当乳糖首次出现在环境中时，这些通透酶能帮助细胞感知并摄入乳糖。
6. The lac Operon: When Lactose Is Present | lac 操纵子：乳糖存在时
When lactose is present in the environment, it acts as an inducer. Lactose is converted into allolactose, which binds to the lac repressor protein. This binding causes a conformational change in the repressor, altering its shape so that it can no longer bind to the operator sequence. The repressor falls off the operator, and RNA polymerase is now free to transcribe the structural genes.
当环境中存在乳糖时，乳糖作为诱导物发挥作用。乳糖被转化为异乳糖，异乳糖与 lac 阻遏蛋白结合，导致阻遏蛋白发生构象变化，使其无法再与操纵基因序列结合。阻遏蛋白从操纵基因上解离下来，RNA 聚合酶即可自由转录结构基因。
Lactose present → Allolactose binds repressor → Transcription proceeds
乳糖存在 → 异乳糖结合阻遏蛋白 → 转录正常进行
The structural genes are then transcribed as a single polycistronic mRNA molecule, which is translated into the three enzymes: β-galactosidase, permease, and transacetylase. These enzymes enable the bacterium to import and metabolise lactose rapidly. Once lactose is depleted, allolactose levels drop, the repressor regains its active conformation, and it again binds to the operator to switch off the operon.
随后，结构基因被转录为一条多顺反子 mRNA，并翻译为三种酶：β-半乳糖苷酶、通透酶和转乙酰酶。这些酶使细菌能够快速摄取并代谢乳糖。当乳糖耗尽后，异乳糖水平下降，阻遏蛋白恢复其活性构象，重新结合操纵基因，关闭操纵子。
It is important to note that the lac repressor is not an all-or-nothing switch in the presence of lactose; rather, the inducer-repressor interaction is dynamic. However, for A-Level purposes, it is sufficient to describe the system as a simple on/off switch.
需要注意的是，在乳糖存在时，lac 阻遏蛋白并非一个绝对的开关；诱导物与阻遏蛋白的结合实际上是动态的。但对于 A-Level 考试而言，将该系统描述为简单的开/关模式已经足够。
7. Catabolite Repression: The Role of Glucose and cAMP-CAP | 分解代谢物阻遏：葡萄糖与 cAMP-CAP 的作用
The lac operon is not controlled solely by the presence or absence of lactose. It is also subject to catabolite repression, which ensures that glucose, the preferred energy source, is used preferentially over lactose. This is a form of positive control mediated by the catabolite activator protein (CAP) and cyclic AMP (cAMP).
lac 操纵子并非仅受乳糖存在与否的调控，它还受到分解代谢物阻遏的调控。这一机制确保葡萄糖（首选能源）优先于乳糖被利用。这是一种由分解代谢物激活蛋白（CAP）和环状 AMP（cAMP）介导的正调控。
When glucose is abundant, the intracellular concentration of cAMP is low. CAP cannot bind to its binding site near the promoter without cAMP. Therefore, RNA polymerase binds to the lac promoter with low efficiency, and the lac operon is transcribed at a very low rate, even if lactose is present. This makes biological sense: why bother metabolising lactose when glucose is available?
当葡萄糖充足时，细胞内 cAMP 浓度较低。没有 cAMP，CAP 无法结合到启动子附近的结合位点。因此，RNA 聚合酶以较低的效率结合 lac 启动子，即使乳糖存在，lac 操纵子的转录速率也非常低。这在生物学上是合理的：既然有葡萄糖可用，何必费力代谢乳糖呢？
When glucose is depleted, cAMP levels rise. cAMP binds to CAP, causing a conformational change that allows CAP to bind to the CAP-binding site on the DNA. This binding bends the DNA and increases the affinity of RNA polymerase for the promoter, significantly enhancing transcription of the lac operon.
当葡萄糖耗尽时，cAMP 水平升高。cAMP 与 CAP 结合，引起构象变化，使 CAP 能够结合 DNA 上的 CAP 结合位点。这种结合使 DNA 发生弯曲，增强了 RNA 聚合酶对启动子的亲和力，从而显著提高 lac 操纵子的转录水平。
Glucose low → cAMP high → cAMP-CAP complex activates transcription
葡萄糖低 → cAMP 高 → cAMP-CAP 复合物激活转录
Thus, the lac operon is controlled by two regulatory mechanisms: a negative control by the lac repressor (responding to lactose) and a positive control by cAMP-CAP (responding to glucose). The operon is fully expressed only when lactose is present AND glucose is absent.
因此，lac 操纵子受两种调控机制控制：lac 阻遏蛋白的负调控（响应乳糖）和 cAMP-CAP 的正调控（响应葡萄糖）。只有当乳糖存在且葡萄糖缺乏时，lac 操纵子才会被充分表达。
| Condition | 条件 | Repressor | 阻遏蛋白 | CAP-cAMP | CAP-cAMP | Transcription Level | 转录水平 |
| Glucose present, lactose absent | Active (binds operator) | Inactive | Very low / off |
| Glucose present, lactose present | Inactive (bound by allolactose) | Inactive | Low |
| Glucose absent, lactose present | Inactive (bound by allolactose) | Active | High |
8. The trp Operon: A Repressible Operon | trp 操纵子：阻遏型操纵子
The trp operon in E. coli is the classic example of a repressible operon. It controls the biosynthesis of tryptophan, an amino acid. The operon contains five structural genes (trpE, trpD, trpC, trpB, and trpA) that code for the enzymes required for tryptophan synthesis. Unlike the lac operon, which is normally off, the trp operon is normally on, producing tryptophan whenever the amino acid is needed.
大肠杆菌的 trp 操纵子是阻遏型操纵子的经典范例，控制色氨酸（一种氨基酸）的生物合成。该操纵子包含五个结构基因（trpE、trpD、trpC、trpB 和 trpA），编码色氨酸合成所需的酶。与通常关闭的 lac 操纵子不同，trp 操纵子通常处于开启状态，在需要时持续合成色氨酸。
When tryptophan is abundant in the environment, the cell does not need to synthesise it. In this case, tryptophan acts as a corepressor. It binds to the trp repressor protein, which is synthesised in an inactive form. The tryptophan-repressor complex undergoes a conformational change that allows it to bind to the operator sequence of the trp operon. This binding blocks RNA polymerase and stops transcription, preventing the wasteful synthesis of enzymes that are no longer needed.
当环境中色氨酸充足时，细胞无需合成色氨酸。此时，色氨酸作为辅阻遏物，与 trp 阻遏蛋白（其合成时处于非活性形式）结合。色氨酸-阻遏蛋白复合物发生构象变化后，能够结合 trp 操纵子的操纵基因序列。这种结合阻断 RNA 聚合酶，使转录停止，从而避免浪费性合成不再需要的酶。
Tryptophan high → Tryptophan binds repressor → Transcription blocked
色氨酸高 → 色氨酸结合阻遏蛋白 → 转录被阻断
When tryptophan levels are low, the repressor is inactive and cannot bind to the operator. RNA polymerase freely transcribes the structural genes, and tryptophan is synthesised. In this way, the trp operon is turned off precisely when its product is no longer needed.
当色氨酸水平较低时，阻遏蛋白处于非活性状态，无法结合操纵基因。RNA 聚合酶可以自由转录结构基因，色氨酸得以合成。通过这种方式，trp 操纵子恰好在其产物不再需要时被关闭。
9. Attenuation: An Additional Level of Control in the trp Operon | 衰减调控：trp 操纵子的额外调控层级
In addition to the repressor-operator system, the trp operon is also controlled by a mechanism called attenuation. This process occurs during transcription and depends on the formation of alternative secondary structures in the 5′ region of the trp mRNA. Attenuation is particularly relevant in the trp operon of E. coli and is a popular A-Level extension topic.
除了阻遏蛋白-操纵基因系统之外，trp 操纵子还受一种称为衰减调控的机制控制。该过程发生在转录过程中，依赖于 trp mRNA 5′ 端区域替代性二级结构的形成。衰减调控在大肠杆菌 trp 操纵子中尤为典型，属于 A-Level 常考的拓展内容。
The leader sequence of the trp mRNA contains four regions (1-4) that can pair with each other in different ways. When tryptophan is abundant, ribosomes rapidly translate region 1, which contains two adjacent tryptophan codons. This prevents region 1 from pairing with region 2. Instead, region 3 pairs with region 4, forming a terminator hairpin that causes RNA polymerase to detach prematurely. Transcription is terminated before the structural genes are transcribed.
trp mRNA 的先导序列包含四个区段（1-4），它们可以以不同方式相互配对。当色氨酸充足时，核糖体快速翻译含有两个连续色氨酸密码子的区段 1，阻止区段 1 与区段 2 配对。此时，区段 3 与区段 4 配对，形成终止子发夹结构，导致 RNA 聚合酶提前脱离，转录在结构基因被转录之前就终止了。
When tryptophan is scarce, the ribosome stalls at the tryptophan codons in region 1. Region 2 then pairs with region 3, forming an anti-terminator hairpin. Region 4 remains unpaired, so no terminator structure forms. RNA polymerase continues transcription, and the structural genes are expressed.
当色氨酸缺乏时，核糖体在区段 1 的色氨酸密码子处停滞。区段 2 与区段 3 配对，形成反终止子发夹结构。区段 4 保持未配对状态，因此不会形成终止子结构。RNA 聚合酶继续转录，结构基因得以表达。
Tryptophan high → Terminator hairpin forms → Transcription stops
色氨酸高 → 形成终止子发夹 → 转录停止
Tryptophan low → Anti-terminator forms → Transcription continues
色氨酸低 → 形成反终止子 → 转录继续
Thus, the trp operon is regulated at two levels: by the repressor-operator system (which prevents transcription initiation) and by attenuation (which controls transcription elongation). This dual control provides a fine-tuned response to changing tryptophan concentrations.
因此，trp 操纵子在两个水平上受到调控：阻遏蛋白-操纵基因系统（控制转录起始）和衰减调控（控制转录延伸）。这种双重调控使细胞能够对色氨酸浓度的变化做出精细响应。
10. Comparing the lac and trp Operons | lac 操纵子与 trp 操纵子的比较
Understanding the differences between the lac and trp operons is essential for A-Level exams. The table below summarises the key similarities and differences.
理解 lac 操纵子与 trp 操纵子之间的差异对 A-Level 考试至关重要。下表总结了它们的主要异同。
| Feature | 特征 | lac Operon | lac 操纵子 | trp Operon | trp 操纵子 |
| Type | 类型 | Inducible | 诱导型 | Repressible | 阻遏型 |
| Default state | 默认状态 | Off | 关闭 | On | 开启 |
| Regulatory molecule | 调控分子 | Allolactose (inducer) | 异乳糖（诱导物） | Tryptophan (corepressor) | 色氨酸（辅阻遏物） |
| Repressor initial state | 阻遏蛋白初始状态 | Active | 活性 | Inactive | 非活性 |
| Function | 功能 | Catabolic (break down lactose) | 分解代谢（分解乳糖） | Anabolic (synthesise tryptophan) | 合成代谢（合成色氨酸） |
| Additional control | 额外调控 | cAMP-CAP (catabolite repression) | cAMP-CAP（分解代谢物阻遏） | Attenuation | 衰减调控 |
Inducible operon: usually OFF, turned ON by inducer
诱导型操纵子：通常关闭，由诱导物开启
Repressible operon: usually ON, turned OFF by corepressor
阻遏型操纵子：通常开启，由辅阻遏物关闭
11. Binding of Regulatory Proteins: Molecular Details | 调控蛋白的结合：分子细节
At the molecular level, regulatory proteins exert their effects through specific interactions with DNA. The lac repressor and trp repressor are both allosteric proteins: they have two distinct binding sites, one for the operator DNA and one for the small molecule (inducer or corepressor). Binding of the small molecule causes a conformational change that alters the protein’s affinity for DNA.
在分子水平上，调控蛋白通过与 DNA 的特异性相互作用来发挥功能。lac 阻遏蛋白和 trp 阻遏蛋白都是变构蛋白：它们具有两个不同的结合位点，一个结合操纵基因 DNA，另一个结合小分子（诱导物或辅阻遏物）。小分子的结合引起构象变化，从而改变蛋白对 DNA 的亲和力。
For the lac repressor, binding of allolactose decreases its affinity for the operator, causing it to dissociate. For the trp repressor, binding of tryptophan increases its affinity for the operator, allowing it to bind and block transcription. This elegant design allows the same basic mechanism (allosteric regulation) to generate opposite outcomes.
对于 lac 阻遏蛋白，结合异乳糖后其与操纵基因的亲和力下降，从而解离下来。对于 trp 阻遏蛋白，结合色氨酸后其与操纵基因的亲和力增加，使其能够结合并阻断转录。这种精巧的设计使相同的基本机制（变构调控）能够产生相反的调控结果。
RNA polymerase itself is also a key player in regulation. In the lac operon, the cAMP-CAP complex helps recruit RNA polymerase to the promoter and stabilises its binding, effectively enhancing transcription initiation. This is an example of positive regulation, as opposed to the negative regulation exerted by repressors.
RNA 聚合酶本身也是调控中的关键角色。在 lac 操纵子中，cAMP-CAP 复合物帮助招募 RNA 聚合酶到启动子并稳定其结合，从而有效增强转录起始。这是正调控的典型例子，与阻遏蛋白施加的负调控形成对比。
12. Exam Focus: Common Questions and Key Points | 考点聚焦：常见问题与关键要点
In CIE A-Level Biology examinations, questions on prokaryotic gene regulation often require students to explain the roles of specific components, predict the effects of mutations, or compare the lac and trp operons. The following points are particularly important for exam success.
在 CIE A-Level 生物学考试中，原核基因调控的题目通常要求学生解释各组件的功能、预测突变的影响，或比较 lac 与 trp 操纵子。以下要点对考试取得高分尤为重要。
- Define key terms precisely: Promoter, operator, structural gene, regulatory gene, repressor, inducer, corepressor, and allosteric regulation.
- Explain the lac operon in both states: Describe what happens when lactose is present and absent, including the role of allolactose.
- Explain catabolite repression: Describe how glucose regulates the lac operon through cAMP and CAP.
- Explain the trp operon: Describe how tryptophan acts as a corepressor to switch off the operon.
- Compare and contrast: Be able to state at least three differences between the lac and trp operons.
- 精确定义关键术语：启动子、操纵基因、结构基因、调控基因、阻遏蛋白、诱导物、辅阻遏物和变构调控。
- 解释 lac 操纵子的两种状态：描述乳糖存在和不存在时发生的事件，包括异乳糖的作用。
- 解释分解代谢物阻遏：描述葡萄糖如何通过 cAMP 和 CAP 调控 lac 操纵子。
- 解释 trp 操纵子：描述色氨酸如何作为辅阻遏物关闭操纵子。
- 比较与对比：能够说出 lac 和 trp 操纵子之间的至少三个差异。
Mutation analysis: If the lacI gene is mutated so that no functional repressor is produced, the lac operon will be constitutively expressed.
突变分析：如果 lacI 基因发生突变导致无法产生功能性阻遏蛋白，lac 操纵子将被组成型表达。
Mutation analysis: If the operator sequence is mutated so that the repressor can no longer bind, the lac operon will also be constitutively expressed.
突变分析：如果操纵基因序列发生突变导致阻遏蛋白无法结合，lac 操纵子也将被组成型表达。
Students should also practise drawing simple diagrams of the operon in both the ‘on’ and ‘off’ states, labelling all components accurately. These diagrams are often required to gain full marks on extended-response questions.
学生还应练习绘制操纵子在”开启”和”关闭”两种状态下的简单示意图，并准确标注所有组件。在扩展性回答题中，绘制正确的图示往往是获得满分的关键。
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