Negative control of transcription:
Repression
• Name:- Shubham D Maurya
• Roll No: 814
• Subject:- Microbiology Paper 3 (PUSMB-503)

INTRODUCTION
• The bacteria have genomes that contain
thousands of different genes.
• Most of these genes encode proteins,
each with its own role in a process such as
fuel metabolism, maintenance of cell
structure, and defense against viruses.
• Some of these proteins are needed
routinely, while others are needed only
under certain circumstances.
• Thus, cells don't express all the genes in
their genome all the time.

• Expression of a gene (Transcription and translation) is a
highly regulated process.
• Regulation - "Ability of a cell to control or regulate
what proteins it makes from its DNA"
• Regulation of genes in prokaryotes ensures that
a cell's resources are not wasted in
making proteins that the cell does not need at that time.
• Lot of gene regulation occurs at the level
of transcription.
• Positive regulation - Activators
• Negative regulation – Repressors*

• Gene regulation in prokaryotes can be explained with the help of the Operon
model.
• Bacterial genes - Operon
• Operon - a cluster of genes under the control of a single promoter.
• An operon comprises of structural genes, promotor and operator.
• Regulatory sequences
• Regulatory gene - regulatory proteins.
• Operator - Negative regulatory site. Repressor binds here.

Negative Control: Repression
• Negative control of transcription is a control mechanism that prevents
transcription. It includes repression and induction.
• "The enzymes that catalyze the synthesis of a specific product are not made if
the product is already present in the medium in sufficient amounts.
• Example:- Amino Acid Arginine
• The enzymes needed to synthesize the amino acid arginine are made only
when arginine is absent from the culture medium.
• Arginine - Co-repressor – prevents the synthesis of the enzymes – enzyme
repression.

Bacteria growing in a
media without arginine
Time
Relative
Increase
1. Cell Number
2. Total Protein
3. Arginine
Biosynthesis
Enzymes
Arginine Added

Mechanism of
Repression
Transcription of the
arginine genes, which
proceeds when the cell
needs arginine.
1. Expression –
polycistronic mRNA
2. Translation – Enzyme -
synthesised arginine.

When arginine is plentiful it acts as corepressor.
Arginine binds to a specific repressor protein, the arginine repressor, present in the cell.
Repressor binds to the operator, transcription is physically blocked because RNA polymerase can
neither bind nor proceed.
Hence, the polypeptides encoded by the genes in the operon cannot be synthesized.
mRNA is polycistronic, all the polypeptides encoded by this mRNA will be repressed

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