Gene Regulation Complied by Siti Sarah JumaliRoom 14, level 3

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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

Therefore no allolactose

Constitutive expression

RNA pol cannot access the promoter

The lac operon is now repressed

The conformation of the repressor is now altered

Some gets converted to allolactose

Repressor can no longer bind to operator

Translation

The lac operon is now induced

The cycle of lac operon induction and repression

Repressor does not completely inhibit transcription

So very small amounts of the enzymes are made

• 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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