Control of Gene Expression in Prokaryotes. Why regulate gene expression? It takes a lot of energy to make RNA and protein. Therefore some genes active.

Control of Gene Expression in Prokaryotes

Why regulate gene expression?

It takes a lot of energy to make RNA and protein.

Therefore some genes active all the time because their

products are in constant demand.

Others are turned off most of the time and are only switched

on when their products are needed.

Gene Control in Prokaryotes

One way in which prokaryotes control gene expression

is to group functionally related genes together so that

they can be regulated together.

This grouping is called an operon (The clustered genes

are transcribed together from one promoter giving a

polycistronic messenger RNA).

Gene Control in Prokaryotes

The prokaryotic genes organized in to operons.

An operon can be defined as a cluster gene

that encode the proteins necessary to perform

coordinated function. Genes of the same operon

have related functions within the cell and are

turned on (expressed) and off (suppressed)

together.

The first operon discovered was the lac operon

so named because its products are involved in

lactose breakdown.

An operon consists of:

Promoter: binding site for RNA polymerase.

Operator: binding site of repressor, overlaps the promoter.

Structural genes

PromoterThe promoter sequences are recognized

by RNA polymerase, When RNA polymerase

binds to the promoter, transcription occurs.

OperatorRepressor proteins encoded by repressor

genes, are synthesized to regulate gene

expression. They bind to the operator site

to block transcription by RNA polymerase.

Activators

The activity of RNA polymerase is

also regulated by interaction with

accessory proteins called activators.

The presence of the activator

removes repression and

transcription occurs.

Two major modes of transcriptional regulation function in

bacteria (E. coli) to control the expression of operons:

Induction

Repression

Both mechanisms involve repressor proteins.

Induction happen in operons that produce gene products needed

for the utilization of energy.

Repression regulate operons that produce gene products

necessary for the synthesis of small biomolecules such as amino

acids.

Inducible system Also called Positive control

The effector molecule interacts with the repressor protein such that it

cannot bind to the operator.

With inducible systems, the binding of the effector molecule to the

repressor greatly reduces the affinity of the repressor for the

operator as a result the repressor is released and transcription

proceeds.

A classic example of an inducible operon (catabolite-mediated) is

the lac operon, responsible for obtaining energy from galactosides

such as lactose.

Repressible system

Also called Negative control

The effector molecule interacts with the repressor protein such

that it can bind to the operator .

With repressible systems, the binding of the effector molecule

to the repressor greatly increases the affinity of repressor for

the operator, the repressor binds and stops transcription.

A classic example of a repressible (and attenuated) operon is

the trp operon, responsible for the biosynthesis of tryptophan.

Structure of the lac OperonThe lac operon have three structural genes: Z, , Y and A

The z gene codes for β-galactosidase , responsible for the hydrolysis

of the disaccharide, lactose into its monomeric units, galactose and

glucose.

The y gene codes for permease, which increases permeability of the

cell to galactosides.

The a gene encodes a transacetylase.

In addition to the structural genes the lac operon also has regulatory

genes:

Promoter: Binding site for RNA polymerase

Operator: Binding site of repressor

Control of lac operon expression

The control of the lac operon occurs by both positive and negative

control mechanisms.

Negative control of the lac operon

What happens to lac operon when glucose is present and lactose

is absent?

During normal growth on a glucose-based medium (lacking

lactose), the lac repressor is bound to the operator region of the lac

operon, preventing transcription.

What happens when glucose is absent and lactose is present?

The few molecules of lac operon enzymes present will

produce a few molecules of allolactose from lactose.

Allolactose is the inducer of the lac operon.

The inducer binds to the repressor causing a conformational

shift that causes the repressor to release the operator.

With the repressor removed, the RNA

polymerase can now bind the promoter and

transcribe the operon.

Positive Control of the lac operon

What happens when both glucose and lactose levels are high?

Since the inducer is present, the lac operon will be transcribed but

the rate of transcription is very slow (almost repressed) because

glucose levels are high and therefore cAMP levels are low.

The repression of the lac operon under these conditions is termed

catabolite repression and is a result of the low levels of cAMP that

results from an adequate glucose supply.

This repression is maintained until the glucose supply is exhausted.

What happens when glucose levels start dropping in the presence of lactose?

As the level of glucose in the medium falls, the level of cAMP

increases.

Simultaneously the inducer (allolactose) is also binds to the lac

repressor (since lactose is present).

The net result is an increase in transcription from the operon.

The ability of cAMP to activate (increase) expression from the lac

operon results from an interaction of cAMP with a protein termed

CRP (for cAMP receptor protein).

The protein is also called CAP (for catabolite activator protein).

The cAMP-CAP complex binds to a region of the lac

operon just upstream of the promoter. This binding

stimulates RNA polymerase activity 20-to-50-fold.

(Repression of the lac operon is relieved in the

presence of glucose if excess cAMP is added.)

cAMP is therefore an activator of the lac

operon.

This type of regulation by an activator is

positive in contrast to the negative control

exerted by repressors.

trp operonThe trp operon encodes the genes for the synthesis

of tryptophan.As with all operons, the trp operon consists of the

promoter, operator and the structural genes. It is also subject to negative control by a repressor.

In this system, unlike the lac operon, the gene for the repressor is not adjacent to the promoter, but rather is located in another part of the E. coli genome.

Another difference is that the operator resides entirely within the promoter

Unlike an inducible system, the repressible operon is usually turned on.

Structure of the trp operon

The operon consists of:

Five structural genes that code for the three enzymes required

to convert chorismic acid into tryptophan.

A gene (trpL) which functions in attenuation.

Operator

promoter

Gene Gene Function

P/O: Promoter; operator sequence is found in the promoter trp L Leader sequence; containing attenuator (A) sequence the leader

trp E: Gene for anthranilate synthetase subunit

trp D : Gene for anthranilate synthetase subunit

trp C: Gene for glycerolphosphate synthetase

trp B: Gene for tryptophan synthetase subunit

trp A: Gene for tryptophan synthetase subunit

Negative control of trp operon

The affinity of the trp repressor for binding the operator region is

enhanced when it binds tryptophan, blocking further transcription of

the operon and, as a result, the synthesis of the three enzymes will

decline, hence tryptophan is a co-repressor, this means that when

tryptophan is absent expression of the trp operon occurs.

the rate of expression of the trp operon is graded in response to the

level of tryptophan in the cell.

• References:• Brock Biology of microorganisms, 2012.• Molecular Biology of the Cell, Bruce Alberts, 2008.• Molecular cell biology, Darnell, Lodish, and Baltimore

2008.• Review Articles: • Regulation of RNA polymerase I transcription in the nucle

olus - Genes and Develop., 2003.

• Roles of the heat shock transcription factors in regulation of the heat shock response and beyond - FASEB J., 2001.

• Translational Control of Viral Gene Expression in Eukaryotes - Microbiology and Molecular Biology Reviews, 2000.