Control of Gene Expression in Bacteria

What is gene expression?

The process by which a gene product (RNA or protein) is made and becomes active.

Why is gene regulation important in bacteria?

It allows cells to conserve energy and respond to environmental changes.

What model explains gene regulation in bacteria?

The operon model.

What are the three levels of gene regulation in bacteria?

Transcriptional control

Translational control

Post-translational control

Which level of gene regulation in bacteria is most efficient? Why?

Transcriptional control (prevents unnecessary protein production).

Which level of gene regulation in bacteria is the fastest?

Post-translational control (activates existing proteins quickly).

What does the lac operon do?

Controls genes needed to metabolize lactose.

What are the key genes in the lac operon?

lacZ → β-galactosidase

lacY → permease

lacA → transacetylase

What is the function of lacI?

Codes for the repressor protein.

What is negative control?

A repressor binds DNA and blocks transcription.

What happens when lactose is absent?

Repressor binds operator → transcription OFF.

What happens when lactose is present?

Lactose inactivates repressor → transcription ON.

What is positive control?

Activator protein enhances transcription.

What is CAP (Catabolite Activator Protein) and how is it activated?

a transcriptional activator protein in E. coli that boosts the expression of genes used to break down alternative sugars (like lactose) when glucose is low

binding to cyclic AMP (cAMP) and then to DNA, helping RNA polymerase initiate transcription

What happens when glucose is high?

CAP inactive → lac operon transcription reduced.

What is an inducer?

A molecule (like lactose) that activates transcription.

initiates gene expression by binding to a repressor protein, inactivating it, and allowing transcription to proceed

What is catabolite repression?

Glucose prevents lac operon activation even if lactose is present.

lacZ⁻ mutation effect?

Cannot break down lactose.

lacY⁻ mutation effect?

Cannot import lactose.

lacI⁻ mutation effect?

Genes expressed constitutively (always ON).

What is an operon?

A group of genes transcribed together under one promoter.

What are key parts of an operon?

Promoter

Operator

Structural genes

What does the trp operon control?

Tryptophan synthesis.

What type of control is it trp operon?

Negative feedback repression.

What is a co-repressor?

Molecule (tryptophan) that activates repressor

When is trp operon ON?

Low tryptophan.

When is it OFF?

High tryptophan

What is a regulon?

A group of genes regulated by the same protein.

Example of regulon?

SOS response (DNA repair).

What would be the consequence of a mutation that rendered the lac repressor unable to bind to lactose?

a) the lac genes would never be expressed

b) the lac genes would be expressed only when lactose is present and glucose is high

c) the lac genes would be expressed constitutively

d) the lac genes would be expressed whenever lactose is present

e) the lac genes would be expressed only in the absence of lactose

a) the lac genes would never be expressed

What would be the consequence of a mutation that rendered the trp repressor unable to bind to the tryptophan?

a) the trp genes would never be expressed

b) the trp genes would be expressed only when tryptophan is present and glucose is absent

c) the trp genes would be expressed constitutively

d) the trp genes would be expressed whenever tryptophan is present

e) the trp genes would be expressed only in the absence of tryptophan

c) the trp genes would be expressed constitutively

An ______ is a stretch of DNA consisting of an operator, a promoter, and genes for a related set of proteins, usually making up an entire metabolic pathway.

operon

The _________ are arranged sequentially after the promoter.

genes of an operon

A _______ is a specific nucleotide sequence in DNA that binds RNA polymerase, positioning it to start transcribing RNA at the appropriate place.

promoter

A _______ codes for a protein, such as a repressor, that controls the transcription of another gene or group of genes.

regulatory gene

Regulatory proteins often bind to the ______ to control expression of the operon.

operator

A ______ is a protein that inhibits gene transcription. In bacteria, this protein binds ot the DNA in or near the promoter.

repressor

An ______ is a specific small molecule that binds to a bacterial repressor protein and changes its shape so that it cannot bind to an operator, thus switching an operon on.

inducer

The lac operon is regulated by both the availability of lactose and glucose. How does E. coli regulate transcription of the lac operon?

operon not transcribed

lactose absent

The lac operon is regulated by both the availability of lactose and glucose. How does E. coli regulate transcription of the lac operon?

operon transcribed at a low rate

lactose present, glucose present

The lac operon is regulated by both the availability of lactose and glucose. How does E. coli regulate transcription of the lac operon?

operon transcribed at a high rate

lactose present, glucose absent