Gene regulation controls which genes are expressed and at what levels. In bacteria, regulation commonly occurs at the transcription initiation step. Regulation happens through operons, which are clusters of genes controlled by a single promoter. Operons contain a promoter where RNA polymerase binds to initiate transcription. They also contain an operator site where a regulatory protein can bind to control gene expression. These regulatory proteins respond to extracellular signals and can act as activators to increase transcription or repressors to decrease transcription. Some operons are inducible, meaning a small molecule can turn them on, while others are repressible and turned on by default but can be turned off. The lac operon is an example of an inducible operon in E. coli regulated by an
2. What is Gene Regulation?
• It can be defined as
the mechanisms for controlling
which genes get expressed and at
what levels.
• There are various stages at which
expression of a gene can be
regulated.
• But the most common stage of
regulation in bacteria is a
transcription initiation step.
3. OPERON
• Regulation in prokaryotes
occurring through a
functioning unit of genomic
DNA known as operon.
• Which contains a cluster of
a gene under the control of
a single promotor.
4. OPERON
• A unit of coordinated and regulated gene activity found in
prokaryotes, by means of which the control of the synthesis of a
protein or a group of protein is determined.
• It consists of a segment of genomic DNA containing a structural
genes promotor and operator together with one or more
regulatory regions
5. Promoter
• The region at the start of a gene where RNA polymerase binds and
initiates transcription.
• A cluster of genes under control of a single promoter is known as
an operon.
6. operator
• A DNA sequence within the operon to which a specific
repressor protein can bind , thereby regulating the
functioning of a structural gene or group of gene.
7. Regulatory proteins
• Genes are often control by extracellular signals in case of bacteria
thus means molecule present in the growth medium.
• These signals are communicated to genes by regulatory proteins
which come into types;
• Positive regulation or activators which increase the transcription
of the regulator gene.
• Negative regulators or repressors which decrease or eliminate
transcription .
8. Activators Vs Repressors
An activator increases
transcription of the operon (e.g.,
by helping RNA polymerase bind
to the promoter).
A repressor reduces
transcription (e.g., by blocking
RNA polymerase from moving
forward on the DNA).
9. INDUCIBLE & REPRESSIBLE OPERONS
• Regulatory proteins often bind to small molecules, which can
make the protein active or inactive by changing its ability to bind
DNA.
• Some operons are inducible, meaning that they can be turned on
by the presence of a particular small molecule.
• Others are repressible, meaning that they are on by default but
can be turned off by a small molecule.
10. The lac operon
• One of the inducible operon
• The lac genes of E.coli are transcribed
from a promoter that is regulated by an
activator and a repressor working in the
simple way .
• The three lac genes are arranged
adjacently and together called the lac
operon. The lac promoter located at the
5’ end of lacZ, directs transcription of all
there genes as a single mRNA.
