Gene Regulation
Complied by Siti Sarah Jumali
Room 14, level 3
ext2123
Overall process of transcription and
translation
Regulation of bacterial expression
• Most microbial metabolic reactions require
enzymes
• Some enzymes are needed in large amount
throughout the bacterial life as a living demand
i.e pyruvate dehydrogenase in glycolysis
• In other cases, the enzyme were only needed in
certain amount.
• This is when the operon system comes into play
Repression
• Inhibits gene expression and decreases
enzyme synthesis
• Prevent overabundance of and end product of
a metabolic pathway
• The protein used to decrease the rate of
production is known as repressor
• It has the ability to block RNA polymerase
• The default position of repressible gene is
turned on
Induction
• Turns on the transcription of a gene
• The substance involve is known as inducer
• The enzymes which are snthesized in the
presence of inducers are termed inducible
enzymes
• Eg. The enzyme β-galactosidase that splits
lactose into glucose and galactose for E. coli.

Regulatory proteins have
two binding sites


One for a small effector
molecule
The other for DNA
The Operon model of expression
• Describes the regulation of protein expression
• Genes that determine the surface of protein is
known as structural genes
• In lac operon, there are 2 short DNA segment
known as promoter and operator
Terminologies
• Promoter – region of DNA where RNA
polymerase initiate transcription
• Operator – acts as the traffic light that
instructs the structural genes which are going
to be transcribed
• Operon – consists of operator, promoter and
three structural genes
RNA pol cannot
access the
promoter
Constitutive
expression
Therefore no allolactose
The lac operon is now repressed
The lac operon is now
induced
Translation
The conformation of the
repressor is now altered
Repressor can no longer
bind to operator
Some gets converted to allolactose
Repressor does not completely
inhibit transcription
So very small amounts of the
enzymes are made
The cycle of lac operon induction and repression
• Example of positive
control
• When cAMP binds to
CAP, complex binds to
CAP site near lac
promoter
• Resulting bend in DNA
enhances RNA
polymerase binding
which increases
transcription
• When both lactose and glucose are high, the lac
operon is shut off
– Glucose uptake causes cAMP levels to drop
– CAP does not activate transcription
– Bacterium uses one sugar at a time, glucose
• When lactose is high and glucose is low, the lac
operon is turned on
– Allolactose levels rise and prevent lac repressor from binding to
operator
– CAP is bound to the CAP site
– Bacterium uses lactose
• When lactose is low and glucose is high or low, the
lac operon is shut off
– Under low lactose conditions, lac repressor prevents
transcription of lac operon
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