Theme 3 Module 2: Prokaryotic Transcriptional Regulation

This module explores prokaryotic transcriptional regulation.
Covers how bacterial cells organize related genes, negative and positive gene expression regulation, and how environmental cues regulate gene expression.

E. coli metabolizes glucose before lactose when both are present.
The shift from glucose to lactose metabolism is tightly regulated at the transcriptional level.
E. coli detects changes in glucose levels and the presence of lactose as environmental cues.

As E. coli transitions, there's an increase in beta-galactosidase and lactose permease proteins.
These proteins are undetectable when glucose is the primary nutrient source.
Glucose potentially inhibits the expression of these gene products, while lactose may induce their expression after glucose depletion.
Lactose permease is a transport protein for importing lactose.
Beta-galactosidase cleaves lactose into glucose and galactose inside the cytoplasm.
The expression of these proteins is linked due to their functional relationship.

Discovered in 1961 by Francois Jacob and Jacques Monod.
Functionally related genes in bacteria are organized into transcriptional units (operons).
Operons are coordinately controlled by a single "on-off switch".
A bacterial operon consists of a promoter, an operator, and a cluster of genes.
The operator is a nucleotide sequence that can allow or inhibit transcription.
When the operator is not bound by an inhibitor, RNA polymerase can transcribe the genes.
Transcription results in a polycistronic mRNA that codes for multiple proteins.
The polycistronic mRNA contains start and stop codons for each polypeptide.

The lac operon is a model for transcriptional regulation in prokaryotes.
Regulatory sequences include the promoter and the operator (lacO).
The lacI gene codes for a repressor protein that binds to the operator.
Structural genes include lacY (lactose permease) and lacZ (beta-galactosidase).
The operator allows transcription until turned off by a repressor.
lacI codes for a repressor protein that inhibits transcription, known as negative transcriptional regulation.

A repressor protein binds to the operator, turning off transcription.
The lac operon is negatively regulated.
The repressor protein, encoded by lacI, is constitutively expressed at low levels.
The repressor binds to the lacO operator, preventing RNA polymerase from binding to the promoter.
This is typical when E. coli cells are exposed to glucose.
In this state, no beta-galactosidase or lactose permease is produced.

The lac operon repressor inhibits the expression of beta-galactosidase and lactose permease.
The repressor is a tetrameric protein, made of four identical subunits, that binds tightly to the operator.
Binding of the repressor protein twists the DNA into a loop, preventing RNA polymerase from binding to the promoter.
The presence of glucose facilitates the constitutive expression of the repressor protein.

Negative transcriptional regulation by the repressor can be allosterically inhibited.
When glucose is depleted and lactose is present, lactose acts as an inducer molecule.
Lactose binds to specific sites on the repressor protein, causing a conformational change.
This change prevents the repressor from binding to the operator.
RNA polymerase can then attach to the promoter and transcribe the genes to produce beta-galactosidase and lactose permease.

In the absence of glucose, positive regulation of the lac operon promotes the production of beta-galactosidase and lactose permease.
Decreased glucose levels lead to increased intracellular cAMP.
This increase in cAMP levels contributes to the positive regulation of the lac operon.

cAMP concentration indicates the nutritional state of E. coli cells.
High glucose levels inhibit adenylyl cyclase, reducing cAMP production, resulting in low intracellular cAMP levels.
Low glucose levels increase adenylyl cyclase activity, leading to high cAMP levels.

Positive regulation involves CRP (cAMP Receptor Protein) or CAP (Catabolite Activator Protein).
cAMP binds to CRP/CAP, which then binds to a specific site on the bacterial DNA.
The amount of intracellular cAMP determines the degree of positive regulation.
cAMP binds to CRP as an allosteric activator, inducing a conformational change.
The CRP-cAMP complex binds to the DNA and activates the transcription of beta-galactosidase and lactose permease in the presence of lactose.
Low glucose levels signal an increase in cAMP to activate the positive regulator (CRP-cAMP), and the presence of lactose binds the repressor protein (LacI).

High extracellular glucose inhibits adenylyl cyclase, resulting in low cAMP levels.
Even in the presence of lactose, the CRP-cAMP complex will not bind the lac operon, leading to lower levels of transcription.
E. coli cells preferentially utilize all glucose before utilizing lactose.

Prokaryotic transcription can be negatively and positively regulated.
An operator can turn on transcription until it is turned off.
Negative regulation involves a repressor protein binding to the operator, turning off transcription.
Inducer proteins bind to repressor proteins, preventing their binding to the DNA.
Positive regulation involves a transcriptional activator protein binding to an activator binding site on the DNA.
This allows for the recruitment of RNA polymerase to the promoter and the initiation of transcription.

E. coli cells detect environmental cues that facilitate the transition from glucose to lactose metabolism.
The lac operon uses a two-part control mechanism (negative and positive regulation).
Lactose activates, while glucose represses, the expression of lac operon gene products.
Control occurs at the level of turning transcription on and off.

High levels of lac operon mRNA are found when bacteria are actively utilizing lactose.
Low levels are seen during the transition from glucose to lactose utilization.
No lac operon mRNA is present when bacteria are actively utilizing glucose.

Functionally related genes are grouped into operons in prokaryotes.
Transcription in prokaryotes can be negatively or positively regulated.
Prokaryotic transcriptional regulation is dependent on responses to changing environmental conditions.