TOPIC: GENE REGULATION IN PROKARYOTES
Different genes in an organism are meant for the
synthesis of different proteins.
All these proteins are not needed at one time.
The coded information present in DNA is expressed by
the synthesis of specific proteins’
However,some genes are constantly transcribed and
expressed in all cells at all time,because their product
are constantly needed in all cells.
Such genes are called constitutive genes or house
But some genes expressed only in some type of cells or
on special occasion.
Such genes are called non constitutive or luxury
The contol of gene expression or protein synthesis is
called gene regulation or it is the process of turning
genes on and off
In prokaryotes, protein synthesis or gene expression is
controlled at transcription level
The regulation may be negative or positive
In –ve regulation an inhibitor or
repressor is present in the cell.The binding of inhibitor
prevents or supresses transcription of relevant gene.
In positive regulation,an inducer
molecule activates the promoter,which promotes
transcription of mRNA.
A negatively regulated system may be either inducible
In inducible gene regulation the structural gene or
genes are induced or ‘switch on’ to produce mRNA.
The substance which induces the gene for protein
synthesis is known as inducer.
The gene complex responsible for the synthesis of
inducible enzymes is called inducible system.
An eg. of repressible system-Trp operon
A transcriptional activator is a protein
that increases transcription of a gene or set of genes.
Most activators are DNA binding proteins that bind
The DNA site bound by the activator is referred to as
an "activator site.
Eg:The catabolite activator protein(CAP; also known
as cAMP repressor protein, CRP) activates
transcription at the lac operon of the bacterium E.coli.
A repressor is a DNA or RNA-binding
protein that inhibits the expression of one or more
genes by binding to the operator.
A DNA-binding repressor blocks the attachment of RNA
polymerase to the promoter.
Thus preventing transcription of the genes into mRNA.
An RNA-binding repressor binds to the mRNA and
prevents translation of the mRNA into protein.
This blocking of expression is called repression.
A corepressor is a molecule that can bind to repressor
and make it bind to the operator tightly, which
A repressor that binds with a corepressor is termed
an aporepressor or inactive repressor.
One type of aporepressor is the trp repressor, an
important metabolic protein in bacteria.
Francis Jacob and Jacques Monad
(1961),explained that gene regulation is by
The operon model for lactose catabolism is called lac
An operon is a group of closely linked structure genes
and associated control genes.
The operon consist of regulatory gene,structural gene,
operator and promoter site.
STRUCTURE OF LAC OPERON
The regulatory gene is the i gene that code for the
This i gene is expressed in all the time hence it is also
known as a constitutive gene.
Three structural genes lac z,lac y,lac a involved in the
synthesis of enzymes for the lactose catabolism.
The 3 genes as transcribed as a polycistronic mRNA.
Lac z code for Betagalactosidase
Lac y code for Lac permease
Lac a code for Transacetylase
Negative control of lac operon
The lac repressor is synthesized through the activity
of lac i gene called the regulator gene.
This repressor is an allosteric protein(A protein whose
shape is changed when it binds a particular molecule)
i)it bind the operator site
ii)it bind the lactose sugar(inducer)
In the absence of inducer,DNA binding site of
repressor is functional.
The repressor protein binds to the DNA at the operator
site of the lac locus and blocks the transcription of lac
genes by RNA polymerase.
The lac enzyme synthesis is inhibited.
Positive control of lac operon
It is an additional regulatory mechanism which allows
the lac operon to sense the presence of glucose,an
alternative and preferred energy source to lactose.
If glucose and lactose are both present,cells will use up
the glucose first.
The presence of glucose in the cell switch off the lac
operon by a mechanism called catabolite reperssion,
which involves a protein called catabolite activator
CAP only binds in the presence of derivative of ATP
called cyclic Adenosine Monophosphate(cAMP)
CAP binds to a DNA sequence upstream of the lac
promoter and enhances binding of the RNA
Transcription of the operon is enhanced
cAMP levels are influenced by glucose.
The enzyme adenylate cyclase catalyze the formation
of cAMP and is inhibited by glucose.
When glucose is available in the cell,adenylate cyclase
is inhibited and cAMP levles are low
Under these condition CAP does not bind upstream of
the promoter and the lac operon is transcribed at very
When glucose is low, adenylate cyclase is not
inhibited, cAMP is higher and CAP binds increasing
the level of transcription.
If lactose and glucose are present together the lac
operon will only transcribed at a low level.