The document discusses gene regulation in prokaryotes using the lac operon in E. coli as an example. The lac operon consists of structural genes that encode proteins for lactose metabolism and regulatory genes that control their expression. In the absence of lactose, a repressor protein binds to the operator region and prevents transcription. When lactose is present, it is converted to allolactose which binds the repressor and prevents it from binding the operator, allowing transcription. The presence of glucose inhibits transcription through a mechanism involving cAMP and the catabolite activator protein.

1. Gene regulation in prokaryotes
LAC OPERON
ELIZABETH PHILIP

2. Gene regulation
Bacterial genomic DNA has many genes that undergoes gene
expression and forms proteins; all these proteins are not needed all
the time; therefore few genes are constantly expressed others are
expressed occasionally.
The control of gene expression or protein synthesis is called gene
regulation or it is the process of turning genes on and off
Gene regulation in prokaryotes is most extensively observed at the
initiation of transcription.
Most genes are controlled by extracellular signals- present in medium.
Repressor, a negative regulatory molecule, binds to the operator gene
and interferes with the expression of genes. Activator, a positive
regulatory molecule, enhances the expression of the genes.

3. The lac operon concept
 Operon : a group or cluster of structural genes under a single promoter; bacterial operons are
polycistronic transcripts that are able to produce multiple proteins from one mRNA
 Francois Jacob and Jacques Monod in 1961
 “Lac operon is an operon or a group of genes with a single promoter that encode genes for the
transport and metabolism of lactose in E.coli and other bacteria.”
 Lac operon concept is an example of prokaryotic gene regulation.

4. STRUCTURE OF LAC OPERON
◦ Lac operon of E.coli consists of structural and the regulatory genes.
◦ a. Structural genes of lac operon:-A cluster of 3 structural genes encoding proteins-
involved in lactose metabolism
 Lac Z – encodes beta galactosidase - breaks down lactose into glucose and galactose
Lac Y – encodes permease - transport lactose into cell
Lac A - encodes transacetylase - addition of acetyl group to galactose
o b.Regulatory genes: includes promoter gene, operator gene, lac I, and catabolite
activator protein (CAP) binding site.
i)The promoter is the binding site for RNA polymerase, the enzyme that performs
transcription.
ii)The operator is a negative regulatory site where the lac repressor protein binds. It is
located between the promoter and the structural genes.

5. iii) Lac I (repressor) gene codes for the lac operon repressor. This gene is located adjacent to
the promoter of the lac operon, with its own promoter and terminator and
is always transcribed, hence the repressor is always synthesized. Repressor binds to the
operator to repress (turn off) the operon
iv)Catabolite Activator Protein (CAP) binding site is a positive regulatory site located just
upstream of the lac operon promoter, where the catabolite activator protein (CAP) binds. The
CAP has binding sites for cAMP and DNA. When cAMP binds CAP, its affinity for the DNA
increases. When bound to DNA, CAP promotes transcription by aiding RNA polymerase bind
to the promoter more efficiently.

7. Mechanism
When lactose is absent:
Control of the lac genes depends on the availability of lactose to the
bacterium.
The lac repressor is synthesized always through the activity of lacI gene.
In the absence of lactose, the repressor protein binds to the DNA at the
operator site and interferes with the binding and transcription by
the RNA polymerase
 Hence, RNA polymerase cannot transcribe the structural genes of
the lac operon and the lactose-metabolising enzymes are not
produced.
Lac operon – TURNED OFF

9. When lactose is present:
◦ When lactose is present ,the structural genes should be transcribed to
produce lactose-metabolising enzymes.
◦ The repressor is prevented from binding to the operator by the action of the
molecule called inducer - allolactose.
◦ The molecule of allolactose is obtained from the lactose (which is now present
in the environment), using the enzyme β-galactosidase (low level of which is
always present in the cell).
◦ Allolactose binds to the repressor protein and causes conformational
change in the repressor due to which, the repressor protein loses its ability to
bind to the operator.
◦ As the repressor do not bind to the operator, the RNA polymerase can bind to
the promoter site and transcribes the genes to further synthesize the lactose-
metabolizing proteins.

11. Regulation of lac operon in the presence of
glucose
• If glucose is present in the medium bacteria will take up glucose
first.
• The presence of glucose in the cell can switch off the lac operon by a
mechanism called catabolite repression, which involves a protein
called catabolite activator protein(CAP)
• CAP (Catabolite Activator Protein) site is present near the promoter
region
• CAP binds to a derivative of ATP called cyclic Adenosine
Monophosphate (cAMP) and is available only in the absence of
glucose

13. Glucose (-) lactose (+)
CAP is functional only when cAMP is bound to it, and cAMP in the cell
is available only in the absence of glucose. Hence CAP allows the transcription of lac
operon genes and subsequent metabolism of lactose, only in absence of glucose.
When no glucose present- cAMP levels increase.
cAMP binds to CAP – forms complex – go and attach to CAP binding site on lac
promoter.
Helps RNA polymerase anchor onto the promoter and the efficiency
of transcription is highly enhanced

15. When glucose is available in the cell, cAMP levles are low.
Without cAMP, CAP cannot bind DNA, which in turn causes
weak binding of RNA polymerase to the lac operon promoter.
 As a result only few transcripts are produced and lactose
utilizing enzymes are not efficiently produced.
 If lactose and glucose are present together the lac operon will
only transcribed at a low level.
Glucose(+) lactose (+)

18. REFERENCE:
◦ https://thebiotechnotes.com/2020/03/20/the-lac-operon/
◦ https://en.wikipedia.org/wiki/Lac_operon
◦ https://youtu.be/CaCq4gglw0g