Gene regulation in prokaryotes, Lac operon, discovery of operon

1. Gene Regulation in Prokaryotes
By
Dr. Akhilesh Kumar
Asst. Professor
Himalayan school of Biosciences
Swami Rama Himalayan University Jollygrant
Dehradun

2. Introduction
Gene regulation refers to the mechanisms that act to induce or repress
the expression of a gene. These include structural and chemical changes to
the genetic material, binding of proteins to specific DNA elements
to regulate transcription, or mechanisms that modulate translation of mRNA.
Bacteria have specific regulatory molecules that control whether a particular gene
will be transcribed into mRNA . Often these molecules act by binding to DNA near the
gene and helping or blocking the transcription enzyme.

3. Operon and its Discovery
In Bacteria genes are often found in ‘Operons’
An operon is a functioning unit of DNA containing a cluster of genes (2-8 genes)
under the control of a single promoter.
French scientist Jacques Monod and Francois Jacob first coined the term ‘operon’ in
a short paper published in 1960 in Proceedings of the French Academy of Sciences.
They elaborated the concept of the operon in several papers that appeared in 1961.
Regulatory Promoter Operator Structural genes
Regulatory gene:- It involved in controlling the expression of one or more other
genes.
Promoter:- Promoter are DNA sequences located in the 5’ region adjacent to the
transcriptional start site. RNA polymerase and accessory proteins (cAMP and CAP)
bind to the promoter to initiate transcription.
Operator:- Operator is a segment of DNA to which repressor binds.
Structural gene:- A Structural gene is a gene that codes for any RNA or protein
product other than a regulatory factor.

4. Lac operon
Lac operon contains three structural genes lacZ (ß-glactosidase), lacY (Permease)
and lacA (Transacetylase). These genes are transcribed a single mRNA (Polycistronic
mRNA), under control of one promoter.
Genes in lac operon specify proteins that help the cell utilize lactose .
 ß-glactosidase enzyme splits lactose into monosaccharide's (glucose and
galactose), Permease allows the cell to import lactose and transacetylase transfers
an acetyl group from acetyl CoA to ß-glactosidase.
Regulatory Promoter Operator lacZ lacY lacA
Structure of lac operon in E. Coli

5. Regulatory Promoter lacZ lacY lacA
Repressor subunits
X X X
No transcription of genes
Repression of lac operon
If lactose is not present in surrounding
medium, then a protein called the lac
repressor binds near the promoter
blocking the action of RNA polymerase
and no transcription.

6. Derepression
Repressor subunits
Regulatory Promoter Operator lacZ lacY lacA
RNA
Polymerase
cAMP
ß-glactosidase Permease Transacetylase
Inactive repressor
The three genes are expressed (produce protein) so long as
RNA polymerase, the enzyme that synthesized RNA, finds its
binding site on the DNA. The binding site for RNA
polymerase is called the promoter. As it happens, the lac
promoter is not the optimal sequence for binding RNA
polymerase, and the protein does not attach to the
promoter stably, unless another protein, cAMP Receptor
Protein (CRP), attaches to its nearby binding site. The
combined presence of CRP and the weak promoter make
stable binding of RNA polymerase much more likely. CRP
binds to its binding site only if the bacterium's favorite sugar,
glucose, is not present. If it is present, then there's no sense
making the proteins encoded by the lac operon, All this is
true if lactose is present in the surrounding medium. Lactose
prevents this by binding to the repressor and changing its
shape so that it cannot attach to DNA. All of this is good:
lactose present means that the repressor does not block
RNA polymerase from transcribing the lac operon, lactose
absent means that RNA polymerase will not waste time
making RNA for protein that won't be used.

7. THANKS