Gene Regulation

Gene Regulation

Complied by Siti Sarah Jumali

Room 14, level 3

ext2123

Overall process of transcription and translation

Gene Expression

An individual’s trillions of body cells all come from one cell (zygote).

In differentiation the cells become different tissues (skin, muscle, organ)

Certain cells will “turn on” or activate only the genes they need. This is called gene expression.

Factors that Affect Gene Expression

The internal and external environment of a cell can influence which genes are “turned on”.

Ex: - Hormones can “tell” a cell to activate a specific gene.

- Outside temperature changes fur color in rabbits.

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

Gene Expression Mechanism

Gene Regulation

• How can genes be turned off and on?• Examples from E. coli

– Inducer – example is lactose (lac operon), pg 187 of (Black, J., 2005)

– Repressor – argenine (arg operon), pg 187-188 of (Black, J., 2005)

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

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

Lac Operon of E. coli

Lac Operon

Induction on Gene Expression

• Turns on the transcription of a gene• The substance involve is known as inducer• The enzymes which are synthesized in the

presence of inducers are termed inducible enzymes

• Eg. The enzyme β-galactosidase that splits lactose into glucose and galactose for E. coli.

Induction - Lac operon

Repression of Gene Expression

• 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 transcription of an operon/RNA polymerase

Repression - Trp operon

Regulatory proteins have two binding sites

One for a small effector molecule

The other for DNA

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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