16.2 Prokaryotic Gene Regulation

Prokaryotic Gene Regulation

Prokaryotic gene regulation allows bacteria to adapt quickly to changing environments by turning genes on or off as needed. Instead of expressing every gene all the time, bacteria use organized gene clusters called operons to coordinate the expression of related genes under a single control system. The trp and lac operons are the two classic examples you need to know, and they illustrate different strategies for controlling transcription.

Prokaryotic Gene Regulation Through Operons

An operon is a cluster of genes controlled by a single promoter, meaning all the genes in the cluster are transcribed together as one mRNA molecule. This setup lets bacteria express a whole set of functionally related genes at once. For example, the lac operon contains all the genes needed for lactose metabolism, and the trp operon contains all the genes needed to build tryptophan.

Every operon has a few key regulatory sequences:

• Promoter: the DNA sequence where RNA polymerase binds to start transcription
• Operator: a DNA sequence located between the promoter and the structural genes where regulatory proteins bind to control whether transcription proceeds

Two main types of regulatory proteins act at the operator:

• Repressors block transcription by binding to the operator and physically preventing RNA polymerase from moving forward. For example, the lac repressor sits on the lac operator when lactose is absent.
• Activators enhance transcription by helping RNA polymerase bind more effectively to the promoter. The CAP-cAMP complex in the lac operon is the key example.

Small molecules called inducers or corepressors fine-tune this system. They bind to regulatory proteins and cause conformational (shape) changes that alter the protein's ability to interact with DNA. Allolactose, for instance, binds to the lac repressor and changes its shape so it can no longer sit on the operator.

Roles in Transcription Control

Repressors work by default in many operons. Without any signal, the repressor binds the operator and blocks transcription. When an inducer molecule is present, it binds to the repressor and changes its shape, causing the repressor to fall off the operator. With the operator now clear, RNA polymerase can transcribe the genes.

Activators take a different approach. Rather than removing a block, they actively recruit or stabilize RNA polymerase at the promoter. Some activators only work when bound to a specific small molecule. In the lac operon, the CAP protein only functions as an activator after cAMP binds to it.

To summarize the key molecules:

• Inducers turn genes on by removing repressors (allolactose removes the lac repressor) or by activating activator proteins (cAMP activates CAP)
• Corepressors turn genes off by activating repressors (tryptophan activates the trp repressor)

Trp vs. Lac Operon Regulation

These two operons are regulated by opposite logic, and understanding that contrast is the fastest way to master this topic.

The trp operon is a repressible system. Its genes (trpE, trpD, trpC, trpB, trpA) encode enzymes that synthesize tryptophan. The operon is on by default because the cell needs to make tryptophan when it's scarce.

1. When tryptophan levels are low, the trp repressor protein is inactive and cannot bind the operator. RNA polymerase transcribes the operon freely.
2. When tryptophan levels are high, tryptophan acts as a corepressor. It binds to the trp repressor, activating it.
3. The active repressor-tryptophan complex binds the operator and blocks transcription.
4. An additional mechanism called attenuation provides a second layer of control by causing RNA polymerase to terminate transcription early when tryptophan is abundant.

The lac operon is an inducible system regulated by both repression and activation. Its genes (lacZ, lacY, lacA) encode enzymes for lactose metabolism. The operon is off by default because the cell doesn't need lactose enzymes unless lactose is actually present.

Repression control:

1. When lactose is absent, the lac repressor binds the operator and blocks transcription.
2. When lactose is present, some lactose is converted to allolactose, which acts as an inducer.
3. Allolactose binds the lac repressor, changing its shape so it releases from the operator.
4. RNA polymerase can now transcribe the lac genes.

Activation control (glucose sensing):

1. When glucose is low, cellular cAMP levels rise.
2. cAMP binds to CAP (catabolite activator protein), forming the CAP-cAMP complex.
3. The CAP-cAMP complex binds near the lac promoter and helps RNA polymerase bind more efficiently, boosting transcription.
4. When glucose is high, cAMP levels drop, CAP is inactive, and transcription stays low even if lactose is present.

Side-by-side comparison:

Additional Regulatory Mechanisms

• Basal transcription refers to the low level of transcription that occurs even without specific activators or inducers. Most promoters allow RNA polymerase to bind at a minimal rate.
• Constitutive expression describes genes that are always transcribed regardless of environmental conditions. Housekeeping genes that the cell constantly needs often fall into this category.
• Feedback inhibition is distinct from gene regulation. It occurs when the end product of a metabolic pathway directly inhibits an enzyme early in that same pathway. This controls enzyme activity, not gene expression, but it works alongside transcriptional regulation to keep metabolic pathways balanced.