Chapter 18: Overview of Gene Regulation, Gene Regulation in Prokaryotes

Gene regulation

Gene regulation

Turning genes on/off as needed

Negative control

Repressor stops transcription

Positive control

Activator helps transcription

Operon

Cluster of genes under one promoter

Lac operon

Lactose digestion

Trp operon

Makes tryptophan

Regulon

Many operons under one control

SOS regulon

DNA repair response

🦠 Why and when a bacterium regulates gene expression

To save energy (don’t make unneeded proteins).

To respond to environment changes (like food sources or stress).

To control growth and reproduction timing.

When:

When nutrients (like sugar) change.

When conditions (temperature, pH, etc.) change.

When it’s stressed (e.g., DNA damage).

Memory trick:

Think “FES” → Food, Environment, Stress.

Those are the main times bacteria adjust their genes.

⚙ Points where gene regulation can happen

Transcriptional control: Deciding whether to start making RNA (most common).

Translational control: Deciding whether to make a protein from mRNA.

Post-translational control: Turning existing proteins on/off.

Memory trick:

Think “Before, During, After” →

Before (transcription),

During (translation),

After (post-translation).

Negative control:

A repressor protein stops gene transcription.

👉 Think: “Repressor = Red light” (stops traffic = stops transcription)

Positive control

An activator protein helps RNA polymerase start transcription.

👉 Think: “Activator = Green light” (go signal for transcription).

What is an operon?

An operon is a group of genes that are controlled together under one promoter (like one on/off switch).

Found in:

🦠 Prokaryotes (bacteria), not eukaryotes.

Memory trick:

Operon = “Operation group” → they work together like a team under one boss (the promoter).

The Lac Operon (Lactose Operon in E. coli)

Helps bacteria digest lactose (milk sugar) only when lactose is present and glucose is low.

Negative control in lac operon:

Repressor protein binds to the operator region and blocks RNA polymerase.

This happens when no lactose is present → gene is OFF.

When lactose appears:

Lactose binds to the repressor, changing its shape so it can’t stick to the operator.

Now RNA polymerase can move → transcription ON.

Memory trick:

No lactose = repressor ON = “No food, no work.”

Lactose present = repressor OFF = “Got food, time to digest!”

Positive control in lac operon:

The CAP protein helps RNA polymerase start transcription.

CAP only works when glucose is low.

CAP binds to CAP site (near the promoter) and helps RNA polymerase stick.

If glucose is high: CAP can’t bind → weak transcription.

Memory trick:

Glucose = easy food.

If glucose is high → don’t bother digesting lactose.

If glucose is low → turn on lac genes fully!

When is lac operon strongly active?

Lactose present (repressor off)

✅ Glucose low (CAP active)

= Strong transcription → bacteria digest lactose fast.

Trp Operon (Tryptophan Operon)

Makes enzymes to produce the amino acid tryptophan.

Negative Control Trp Operon (Tryptophan Operon)

• When tryptophan levels are high, it binds to a repressor, activating it.

• This repressor binds to the operator and stops RNA polymerase → gene OFF.

When tryptophan is low:

• Repressor is inactive → RNA polymerase moves → genes ON → cell makes more tryptophan.

Memory trick:
Tryptophan = “Too much? Turn it off.”
It’s like a thermostat for amino acids.

Global gene regulation:

Cells sometimes need to control many operons at once (like an emergency mode).

Regulon:

A group of operons or genes controlled by the same regulatory protein.

🆘 SOS response regulon

Activated when DNA is damaged.

Normally, LexA repressor keeps DNA repair genes off.

When DNA is damaged, another protein (RecA) inactivates LexA.

This turns many repair genes ON at once → cell fixes DNA.

Memory trick:

SOS = “Save Our Sequence!” → DNA damage emergency response.