📚 Gene Control in Prokaryotes | 原核生物的基因调控
Prokaryotes live in environments where nutrient availability and physical conditions can change abruptly. To survive, they must regulate the expression of genes, switching metabolic pathways on or off rapidly. This control occurs primarily at the level of transcription, allowing cells to avoid wasteful synthesis of enzymes when their substrates are absent. The operon model explains how clusters of functionally related genes are co‑ordinately controlled in bacteria.
原核生物生活在营养供给和物理条件可能急剧变化的环境中。为了生存，它们必须调控基因的表达，快速开启或关闭代谢途径。这种调控主要发生在转录水平，使细胞能够避免在没有底物时浪费性地合成酶。操纵子模型解释了功能相关的基因簇如何在细菌中被协同控制。
1. Gene Control and the Operon Concept | 基因调控与操纵子概念
Gene regulation in prokaryotes enables bacteria to conserve energy and resources by producing enzymes only when they are needed. For example, Escherichia coli does not synthesise the proteins for lactose metabolism unless lactose is present and glucose is absent. This selective expression relies on regulatory proteins that bind to specific DNA sequences near the genes they control.
原核生物的基因调控通过仅在需要时产生酶，使细菌能够节约能量和资源。例如，大肠杆菌只有在乳糖存在且缺乏葡萄糖时，才会合成乳糖代谢所需的蛋白质。这种选择性表达依赖于调控蛋白与其控制基因附近特定 DNA 序列的结合。
The operon, proposed by François Jacob and Jacques Monod in 1961, is a unit of genetic function consisting of a promoter, an operator and structural genes. A regulatory gene, often located outside the operon, codes for a repressor or activator protein. The structural genes are transcribed as a single polycistronic mRNA, which is then translated into a set of enzymes that carry out a common metabolic task.
由 François Jacob 和 Jacques Monod 于 1961 年提出的操纵子是一个遗传功能单位，由启动子、操纵基因和结构基因组成。一个调控基因通常位于操纵子外部，编码阻遏蛋白或激活蛋白。结构基因被转录成一条多顺反子 mRNA，随后翻译出一组执行共同代谢任务的酶。
The operator acts as an on‑off switch: when a repressor binds to the operator, it blocks RNA polymerase from transcribing the structural genes. In many operons, additional proteins such as activators enhance transcription by facilitating polymerase binding to the promoter. This modular design allows bacteria to respond quickly to environmental cues.
操纵基因起着开关的作用：当阻遏蛋白与操纵基因结合时，它会阻止 RNA 聚合酶转录结构基因。在许多操纵子中，激活蛋白等额外蛋白质通过促进聚合酶与启动子的结合来增强转录。这种模块化设计使细菌能够快速响应环境信号。
2. Structure of the lac Operon | 乳糖操纵子的结构
The lac operon of E. coli contains three structural genes: lacZ (encoding β-galactosidase), lacY (encoding lactose permease) and lacA (encoding transacetylase). Upstream of these genes lies a promoter (P) where RNA polymerase binds, and an operator (O) which is the binding site for the lac repressor. A separate regulatory gene, lacI, has its own promoter and codes for the repressor protein.
大肠杆菌的乳糖操纵子包含三个结构基因：lacZ（编码 β-半乳糖苷酶）、lacY（编码乳糖透性酶）和 lacA（编码转乙酰酶）。这些基因的上游有一个 RNA 聚合酶结合的启动子 P 和一个作为 lac 阻遏蛋白结合位点的操纵基因 O。一个独立的调控基因 lacI 拥有自身的启动子，编码阻遏蛋白。
The lac promoter is relatively weak; efficient transcription requires the binding of the catabolite activator protein (CAP) complexed with cyclic AMP (cAMP). The CAP binding site is located just upstream of the promoter. When CAP‑cAMP is bound, it bends the DNA and helps RNA polymerase to start transcription efficiently.
乳糖操纵子的启动子相对较弱；高效的转录需要分解代谢物激活蛋白 CAP 与环腺苷酸 cAMP 的复合物结合。CAP 结合位点恰好位于启动子上游。当 CAP‑cAMP 结合时，它使 DNA 弯曲，帮助 RNA 聚合酶有效地起始转录。
3. Negative Regulation by the lac Repressor | lac 阻遏蛋白的负调控
In the absence of lactose, the lacI gene product, the lac repressor, binds tightly to the operator. This binding physically obstructs the movement of RNA polymerase from the promoter to the structural genes, turning the operon off. The repressor is a tetrameric protein that can simultaneously bind two operators, forming a DNA loop that further represses transcription.
在缺乏乳糖时，lacI 基因产物——lac 阻遏蛋白——会与操纵基因紧密结合。这种结合在物理上阻碍了 RNA 聚合酶从启动子向结构基因移动，从而关闭操纵子。阻遏蛋白是一种四聚体蛋白，能同时结合两个操纵基因，形成 DNA 环，进一步增强阻遏。
The repressor does not permanently stay on the operator. When lactose enters the cell, a small amount is converted into allolactose by β-galactosidase. Allolactose acts as an inducer: it binds to the lac repressor, changing its shape so that the repressor can no longer bind the operator. This release allows RNA polymerase to transcribe the structural genes.
阻遏蛋白并非永久停留在操纵基因上。当乳糖进入细胞，少量会被 β-半乳糖苷酶转化为异乳糖。异乳糖作为诱导物：它与 lac 阻遏蛋白结合，改变其构象，使阻遏蛋白不能再与操纵基因结合。这种解离使 RNA 聚合酶得以转录结构基因。
4. Positive Regulation by CAP‑cAMP | CAP‑cAMP 的正调控
Even with the repressor removed, the lac operon is only transcribed at a low basal level unless CAP is active. The CAP protein requires cAMP as a co‑activator. When glucose levels are high, the enzyme adenylate cyclase is inhibited, causing cAMP levels to be low. Without sufficient cAMP, CAP cannot bind DNA, and the promoter remains largely inaccessible to RNA polymerase.
即使阻遏蛋白被移除，除非 CAP 处于活性状态，乳糖操纵子也只能在低基础水平上转录。CAP 蛋白需要 cAMP 作为辅激活物。当葡萄糖浓度高时，腺苷酸环化酶被抑制，导致 cAMP 水平较低。没有足够的 cAMP，CAP 无法结合 DNA，启动子仍然在很大程度上无法被 RNA 聚合酶接近。
When glucose is scarce, cAMP levels rise, and CAP‑cAMP complexes form. These complexes bind to the CAP site, interacting directly with RNA polymerase and increasing the rate of transcription initiation. This dual control—negative regulation by the repressor and positive regulation by CAP‑cAMP—ensures that the lac operon is fully active only when lactose is present and glucose is absent.
当葡萄糖稀缺时，cAMP 水平上升，形成 CAP‑cAMP 复合物。这些复合物与 CAP 位点结合，直接与 RNA 聚合酶相互作用，提高转录起始速率。这种双向调控——阻遏蛋白的负调控与 CAP‑cAMP 的正调控——确保乳糖操纵子仅在乳糖存在且葡萄糖缺乏时才完全激活。
5. How the lac Operon Functions as an Inducible System | 乳糖操纵子如何作为诱导型系统工作
In the absence of lactose, the repressor is bound to the operator and transcription is blocked. If lactose enters the cell and glucose is available, the repressor is inactivated, but CAP is not bound; thus transcription occurs at a moderate rate. Only when both inducer (allolactose) is present and cAMP levels are high (low glucose) does the lac operon achieve maximum expression.
在没有乳糖时，阻遏蛋白结合在操纵基因上，转录被阻断。如果乳糖进入细胞而葡萄糖可获取，阻遏蛋白被失活，但 CAP 未结合；因此转录以中等速率进行。只有当诱导物（异乳糖）存在且 cAMP 水平高（低葡萄糖）时，乳糖操纵子才达到最大表达。
This behaviour makes the lac operon an inducible operon: its default state is off, and it is turned on by the inducer. Inducible systems typically control catabolic pathways, where a specific substrate triggers the production of the enzymes needed for its own breakdown. The lac operon’s integration of two signals (lactose and glucose) is a classic example of signal integration in prokaryotic gene regulation.
这种行为使乳糖操纵子成为诱导型操纵子：它的默认状态是关闭，由诱导物开启。诱导型系统通常控制分解代谢途径，其中特定底物触发降解自身所需酶的产生。乳糖操纵子对两种信号（乳糖和葡萄糖）的整合是原核生物基因调控中信号整合的经典范例。
6. Overview of the trp Operon | 色氨酸操纵子概述
The trp operon of E. coli contains five structural genes (trpE, trpD, trpC, trpB, trpA) that encode enzymes for the biosynthesis of the amino acid tryptophan. Upstream of these genes is a promoter, an operator and a leader region (trpL). The regulatory gene trpR, located elsewhere, produces an inactive repressor protein that requires tryptophan as a corepressor.
大肠杆菌的色氨酸操纵子包含五个结构基因（trpE、trpD、trpC、trpB、trpA），编码合成色氨酸所需的酶。这些基因的上游有启动子、操纵基因和一个先导区域 trpL。调控基因 trpR 位于别处，产生一种无活性的阻遏蛋白，该蛋白需要色氨酸作为辅阻遏物。
In contrast to the lac operon, the trp operon is a repressible system: its default state is on, and it is turned off when tryptophan is abundant. This makes biological sense because tryptophan synthesis is an anabolic pathway; the cell should only cease producing enzymes for it when the end product is already plentiful.
与乳糖操纵子相反，色氨酸操纵子是一个阻遏型系统：它的默认状态是开启，当色氨酸充足时才被关闭。这在生物学上很合理，因为色氨酸合成是合成代谢途径；细胞只应在终产物已经充足时才停止产生相应的酶。
7. Repression of the trp Operon by Tryptophan | 色氨酸对 trp 操纵子的阻遏
When intracellular tryptophan levels are low, the trp repressor protein does not bind to the operator, because it needs tryptophan to become active. Under these conditions, RNA polymerase transcribes the trp operon continuously, and the biosynthesis enzymes are produced. As tryptophan accumulates, it binds to the repressor as a corepressor, causing a conformational change that enables the repressor–tryptophan complex to attach to the operator.
当胞内色氨酸水平低时，trp 阻遏蛋白不与操纵基因结合，因为它需要色氨酸才能被激活。在此条件下，RNA 聚合酶持续转录色氨酸操纵子，生物合成酶被不断产生。随着色氨酸积累，它作为辅阻遏物与阻遏蛋白结合，引发构象变化，使阻遏蛋白‑色氨酸复合物能够附着在操纵基因上。
Once bound, the complex blocks RNA polymerase from transcribing the structural genes, shutting down enzyme synthesis. However, repression alone reduces transcription only about 70‑fold, which is not as dramatic as in some other operons. A second regulatory mechanism, attenuation, provides additional fine‑tuning in response to tryptophan availability.
一旦结合，该复合物阻止 RNA 聚合酶转录结构基因，从而关闭酶的合成。然而，仅靠阻遏只能使转录下降约 70 倍，不如某些其他操纵子那么显著。第二种调控机制——衰减作用——根据色氨酸的可获取性提供了进一步的精细调节。
8. Attenuation: Fine‑tuning the trp Operon | 衰减作用：精细调节色氨酸操纵子
The trp leader region contains a sequence that can form alternative stem‑loop structures in the mRNA. Immediately after transcription begins, a ribosome attaches to the nascent mRNA and translates a short leader peptide containing two consecutive tryptophan codons. The behaviour of the ribosome determines which RNA secondary structure forms.
色氨酸先导区域包含一段能在 mRNA 中形成替代性茎环结构的序列。转录开始后，核糖体立即附着在新生 mRNA 上，翻译一段含有两个连续色氨酸密码子的短先导肽。核糖体的行为决定了形成哪种 RNA 二级结构。
When tryptophan is abundant, charged tRNATrp is plentiful, and the ribosome quickly translates the leader peptide, moving past the tryptophan codons. This allows the mRNA to fold into a terminator hairpin that causes RNA polymerase to pause and dissociate from the DNA, terminating transcription prematurely. The structural genes are not transcribed.
当色氨酸充足时，负载的 tRNATrp 充足，核糖体能快速翻译先导肽，越过色氨酸密码子。这使得 mRNA 折叠成一个终止发夹，导致 RNA 聚合酶暂停并从 DNA 上脱离，转录提前终止。结构基因不被转录。
In contrast, when tryptophan is scarce, the ribosome stalls at the tryptophan codons due to a shortage of charged tRNATrp. The stalled ribosome masks a different RNA segment, allowing an anti‑terminator structure to form. This anti‑terminator prevents formation of the terminator hairpin, and RNA polymerase continues transcribing the structural genes. Attenuation provides about a further 10‑fold regulation, and together with repression the operon can be controlled over a 700‑fold range.
相反，当色氨酸缺乏时，由于负载 tRNATrp 不足，核糖体在色氨酸密码子处停滞。停滞的核糖体遮盖了不同的 RNA 区段，使得抗终止子结构得以形成。抗终止子阻止了终止发夹的形成，RNA 聚合酶继续转录结构基因。衰减作用提供了额外约 10 倍的调控，与阻遏相结合，该操纵子可在 700 倍的范围内被调控。
9. Comparing Inducible and Repressible Systems | 诱导型与阻遏型系统的比较
The lac and trp operons exemplify two fundamental control strategies in prokaryotes. The table below highlights their key differences.
lac 操纵子和 trp 操纵子代表了原核生物中两种基本的调控策略。下表突出了它们的关键差异。
| Feature | lac Operon | trp Operon |
|---|---|---|
| Type | Inducible | Repressible |
| Pathway controlled | Catabolic (lactose breakdown) | Anabolic (tryptophan synthesis) |
| Default state | Off (repressor bound) | On (repressor inactive) |
| Effector molecule | Inducer (allolactose) | Corepressor (tryptophan) |
| Effect of effector on repressor | Inactivates repressor | Activates repressor |
| Additional regulation | Positive control by CAP‑cAMP | Attenuation at leader region |
In an inducible system, the presence of a substrate triggers enzyme synthesis; in a repressible system, the presence of the end product turns synthesis off. Both strategies use repressor proteins, but the conformational changes induced by effector molecules have opposite outcomes.
在诱导型系统中，底物的存在会触发酶合成；而在阻遏型系统中，终产物的存在则会关闭合成。这两种策略都使用阻遏蛋白，但效应分子诱导的构象变化导致相反的结果。
Moreover, both operons integrate more than one signal. The lac operon monitors lactose and glucose availability via the repressor and CAP, while the trp operon monitors tryptophan levels through both the repressor and the ribosome‑mediated attenuation mechanism. This sophistication allows prokaryotes to maintain cellular economy with remarkable precision.
此外，两种操纵子都整合了不止一种信号。乳糖操纵子通过阻遏蛋白和 CAP 监测乳糖与葡萄糖的可利用性，而色氨酸操纵子则通过阻遏蛋白和核糖体介导的衰减机制监测色氨酸水平。这种精密性使原核生物能以惊人的精度维持细胞经济。
10. Significance of Prokaryotic Gene Control | 原核生物基因调控的意义
Understanding operons is fundamental to molecular biology. The discovery of gene regulation in bacteria earned Jacob and Monod a Nobel Prize and laid the groundwork for recombinant DNA technology. For example, the lac promoter and operator have been used to create inducible expression vectors that allow scientists to switch on production of a desired protein at a chosen time.
理解操纵子是分子生物学的基础。原核生物基因调控的发现使 Jacob 和 Monod 获得了诺贝尔奖，并为重组 DNA 技术奠定了基础。例如，lac 启动子和操纵基因已被用于构建诱导型表达载体，使科学家能在选定的时间开启目标蛋白的生产。
Operon models also inform our understanding of bacterial pathogenicity. Many virulence genes are organised in operons and are tightly regulated by environmental signals—such as temperature or ion concentration—that a pathogen encounters upon entering a host. Interfering with these regulatory circuits is a potential strategy for novel antibiotics.
操纵子模型还加深了我们对细菌致病性的理解。许多毒力基因以操纵子形式组织，并受到病原体进入宿主时遇到的环境信号（如温度或离子浓度）的严格调控。干扰这些调控回路是开发新型抗生素的潜在策略。
Finally, comparative studies of operon structures across different bacterial species reveal evolutionary relationships and show how horizontal gene transfer can assemble new metabolic capabilities. The principles of prokaryotic gene control continue to be a vibrant area of research, with implications for synthetic biology and metabolic engineering.
最后，对不同细菌物种间操纵子结构的比较研究揭示了进化关系，并展示了水平基因转移如何组装出新的代谢能力。原核生物基因调控的原理至今仍是一个活跃的研究领域，并对合成生物学和代谢工程产生深远影响。
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