Gene Expression Regulation and the Lac Operon

These flashcards cover essential vocabulary related to gene expression regulation, mechanisms involved in the lac operon, and related concepts discussed in the lecture.

Gene Expression Regulation

The control of the amount and timing of the appearance of the functional product of a gene.

Operon

A cluster of genes under the control of a single promoter, particularly in prokaryotes.

Lac Operon

A model system for studying gene regulation, specifically for lactose metabolism in E. coli.

Transcriptional Control

Regulation of gene expression at the transcription level, affecting how often a gene is transcribed.

Translational Control

Regulation of gene expression at the translation level, influencing how often mRNA is translated into protein.

Post-Translational Control

Regulation of gene expression after protein synthesis, often involving modifications like phosphorylation.

Inducer

A molecule that increases gene expression by disabling a repressor, such as lactose in the lac operon.

Repressor Protein

A protein that binds to an operator and prevents transcription of the genes in an operon.

Cyclic AMP (cAMP)

A signaling molecule that, when bound to CAP, enhances the transcription of certain operons in low glucose conditions.

Constitutive Genes

Genes that are always expressed at a constant rate.

Catabolite Repression

A mechanism in which the presence of a preferred energy source, like glucose, inhibits the expression of genes responsible for metabolizing other sugars.

Positive Control

A form of regulation where activators enhance the transcription of a gene.

Negative Control

A form of regulation where repressors inhibit the transcription of a gene.

Beta Galactosidase

An enzyme that breaks down lactose into glucose and galactose, produced only when lactose is present.

Galactoside Permease

A membrane protein that facilitates the entry of lactose into the bacterial cell.

Lac Z

The gene in the lac operon that encodes for beta galactosidase.

Lac Y

The gene in the lac operon that encodes for galactoside permease.

Lac I

The regulatory gene in the lac operon that produces the repressor protein.

Allosteric Regulation

Regulation of a protein's function through the binding of a molecule at a site other than the active site, causing a conformational change.

Inducer Exclusion

A mechanism by which the presence of glucose prevents the uptake of other sugars, such as lactose, into the cell.

Replica Plating

A method used to identify mutants by transferring colonies from a master plate to selective media.

Regulatory Gene

A gene that produces regulatory proteins, such as repressors or activators, which control the expression of other genes.

Promoter

A region of DNA where RNA polymerase binds to initiate transcription of a gene.

Gene expression definition

Synthesis and activity of a gene product (RNA or protein) in a cell

When a gene is “expressed”

The gene’s product is produced and functional

Information flow

DNA → mRNA → Protein → Active protein

Regulation stages

Expression can be controlled at transcription, translation, or post-translation

Environmental triggers of expression

pH, temperature, light, nutrients, competition

Internal triggers of expression

Metabolites, regulatory proteins, signaling pathways

Levels of control (overview)

Transcriptional (slow, most efficient); Translational (medium); Post-translational (fastest, least efficient)

Energy vs speed rule

Transcriptional = most energy-efficient; Post-translational = fastest response

Most energy-efficient control

Transcriptional control

Fastest response control

Post-translational control

Constitutive gene

Gene expressed continuously (always “on”); usually essential functions

Regulated gene

Gene whose expression changes (on/off or modulated) with conditions

Model organism for gene control

E. coli (lactose metabolism studies by Jacob & Monod)

β-galactosidase function

Hydrolyzes lactose → glucose + galactose

Lactose role in E. coli

Inducer that triggers β-galactosidase expression when glucose is absent

Sugar preference in E. coli

Glucose preferred; lactose used when glucose is low/absent

Mutagenesis (purpose)

Create random mutations to identify genes required for lactose metabolism

Replica plating (principle)

Colonies replica-transferred to lactose-only medium; non-growers lack lactose-use genes

Replica plating is a microbiological technique used to transfer identical patterns of bacterial colonies from one plate to several others to test their growth under different conditions.

Purpose:

To identify mutant bacteria that differ in metabolic abilities (e.g., can’t metabolize lactose).

How replica plating identifies mutants

Mutants fail to grow on lactose-only plates → defective in lac genes

Operon definition

Cluster of functionally related genes transcribed as a single polycistronic mRNA under one promoter

Polycistronic mRNA

mRNA that encodes multiple proteins (common in bacteria/archaea)

lac operon structural genes

lacZ, lacY, lacA (co-transcribed)

lacZ gene product

β-galactosidase (breaks lactose)

lacY gene product

Galactoside permease (imports lactose)

lacA gene product

Transacetylase (exports/modifies excess sugars)

lacI gene product

Repressor protein (regulates operon; transcribed separately)

Operator (lacO)

DNA site where LacI repressor binds to block transcription

Promoter (lacP)

DNA sequence where RNA polymerase binds to start transcription

A promoter is a specific DNA sequence located just upstream (before) a gene or operon where RNA polymerase bindsto start transcription.

CAP site (lac)

DNA site upstream of promoter where CAP–cAMP binds to activate transcription

Negative control (definition)

Regulatory protein (repressor) prevents transcription

Positive control (definition)

Regulatory protein (activator) increases transcription

lac operon default state

OFF (repressor bound) when lactose absent

Inducer molecule for lac

Allolactose (isomer of lactose) binds LacI and inactivates it

How lactose induces lac operon

Lactose (allolactose) binds LacI → LacI releases operator → RNA Pol transcribes

Outcome when lactose absent

Repressor binds operator → lacZYA not transcribed

CAP (catabolite activator protein)

Activator that recruits/stabilizes RNA polymerase at lac promoter

cAMP role

Binds CAP; CAP–cAMP complex attaches DNA to stimulate transcription

Glucose vs cAMP

High glucose → low cAMP (CAP inactive); Low glucose → high cAMP (CAP active)

Catabolite repression (definition)

High glucose lowers cAMP, reducing CAP activation and lac transcription

Inducer exclusion (definition)

High glucose inhibits lactose import (permease), keeping repressor active

Two ways glucose represses lac

Catabolite repression (↓cAMP) and inducer exclusion (↓lactose entry)

Mechanism	What It Targets	How It Works	Effect on lac Operon

Catabolite Repression

Transcription initiation

High glucose → low cAMP → CAP–cAMP complex cannot form → CAP doesn’t bind promoter → RNA polymerase binds weakly

↓ Transcription (even if lactose present)

Inducer Exclusion

Lactose transport into cell

High glucose → inhibits lactose permease (LacY) → lactose can’t enter → no allolactose → repressor stays bound

↓ Induction (lactose signal blocked)

Highest lac expression condition

Low glucose (↑cAMP, CAP binds) + High lactose (repressor off)

lac expression: High Glc/Low Lac

Low cAMP, CAP off; repressor on → No expression

lac expression: High Glc/High Lac

Low cAMP, CAP off; repressor off → Low expression

lac expression: Low Glc/High Lac

High cAMP, CAP on; repressor off → Maximum expression

lac expression: Low Glc/Low Lac

High cAMP, CAP on; repressor on → No expression

Ara operon purpose

Enables metabolism of arabinose sugar

AraC regulator

AraC + arabinose changes conformation and activates transcription (positive control)

lac vs ara control

Lac uses negative control (repressor off by inducer); Ara uses positive control (activator on with sugar)

Transcriptional control example

LacI blocking RNA polymerase at operator

Translational control example

Altering mRNA stability or ribosome binding (rarer in bacteria)

Post-translational control example

Activating or inhibiting enzymes via phosphorylation or ligand binding

Repressor (definition)

Protein that binds DNA to prevent transcription

Activator (definition)

Protein that binds DNA to increase transcription

Inducer (definition)

Small molecule that inactivates a repressor or activates an activator to turn genes on

Allosteric regulation in lacI

Lactose binding changes LacI shape, reducing DNA affinity

Why bacteria use operons

Coordinate expression of pathway genes efficiently in response to nutrients

Glucose presence effect on lac

Represses lac via ↓cAMP (CAP off) and permease inhibition (inducer exclusion)

Order of regulatory logic for lac

Prefer glucose; only express lactose genes when glucose low and lactose present

RNA polymerase role in operon

Binds promoter; initiates transcription of lacZYA when not blocked

Why transcriptional control is efficient

Prevents unnecessary mRNA and protein synthesis

Why post-translational control is fast

Modifies existing proteins directly without new synthesis

Constitutive lacI expression

Ensures repressor protein is always available

lacI− mutant phenotype

Repressor absent → lac operon expressed even without lactose (constitutive)

Operator (lacO) mutant effect

Repressor cannot bind → lac genes expressed constitutively

CAP− mutant effect

Loss of activation → reduced lac transcription even when lactose is present and glucose is low

cya− (adenylate cyclase) mutant

No cAMP → CAP inactive → low lac transcription unless glucose is absent and other signals compensate

• The cya gene in E. coli encodes adenylate cyclase, the enzyme that makes cAMP (cyclic AMP) from ATP.

• A cya⁻ mutant has a loss-of-function mutation in this gene → no adenylate cyclase activity → the cell cannot produce cAMP.

crp− mutant (CAP missing)

Cannot form CAP–cAMP complex → poor activation of lac promoter

Inducer exclusion mechanism

Glucose transport system inhibits LacY permease, preventing lactose uptake

Why lacA is less emphasized in metabolism

Main roles are detox/export; lacZ and lacY are essential for lactose utilization

Benefit of positive + negative control

Provides fine-tuned response across glucose/lactose combinations

Operon vs regulon

Operon = genes in one transcript; Regulon = multiple operons/genes controlled by same regulator

Example of regulon concept

CAP controls multiple catabolic operons, not just lac

mRNA stability in bacteria

Short half-life enables rapid shutoff of protein synthesis

• When a gene is turned off (transcription stops), the existing mRNA is quickly degraded.

• With no mRNA left, translation stops almost immediately → no more protein synthesis.

So the short mRNA half-life allows the cell to respond rapidly to environmental changes — it can stop making unneeded proteins within minutes.

Ribosome binding site (RBS)

Bacterial Shine–Dalgarno sequence; affects translational control

Global vs local control

Global (e.g., CAP–cAMP) responds to cellular energy; Local (e.g., LacI) responds to specific sugar

Why cells avoid unnecessary enzymes

Conserves ATP, amino acids, and ribosome capacity