Transcription is a process in which mRNA is synthesized from DNA followed by protein synthesis i.e. translation. Regulation is very important to avoid unnecessary expression of gene and to avoid any wastage of cell resources and energy.
Negative control of transcription:
• Name:- Shubham D Maurya
• Roll No: 814
• Subject:- Microbiology Paper 3 (PUSMB-503)
• 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
• Positive regulation - Activators
• Negative regulation – Repressors*
• Gene regulation in prokaryotes can be explained with the help of the Operon
• 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
Bacteria growing in a
media without arginine
1. Cell Number
2. Total Protein
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