Question

• Understand the Trp and Lac operons

Answer 1

Operon is a set of structural genes which are regulated by a common promoter. It is a characteristic feature of prokaryotic organisms.

There are two types of operons, non constitutive operons and constitutive Operons.

Constitutive Operon is the one which is always switched on. It codes for housekeeping genes like gene for ATP synthase enzyme.

Non constitutive Operon includes two types of Operon, inducible Operon and repressible Operons. Inducible Operon is the one which switches on in the present of its starting material and repressible Operon is the one which which of in the presence of its starting material. The example of inducible Operon is lactose Operon and that of repressible Operon is tryptophan Operon.

Structure and functioning of lactose Operon -

CAP is catabolic Activator protein. This is required for the transcription of the lactose Operon. Whenever the levels of glucose in the medium are low, then cyclic AMP is produced. This leads to the formation of CAP cAMP Complex. This binds to the CAP site located in the Operon. This help in the binding of RNA polymerase to the promoter and therefore initiation of transcription.

The promoter is the region on Lac Operon where RNA polymerase binds to initiate the transcription of downstream genes.

The operator is the region on Lac Operon where lactose repressor binds to inhibit the transcription of the Operon. This occurs when the concentration of glucose is high in the medium and concentration of lactose is low. But when the concentration of lactose increases and date of glucose decreases, then lactose molecules act as inducer and bind to lactose repressor. This makes operator free and now RNA polymerase can transcribe the Operon.

LacZ gene encodes for beta galactosidase enzyme. This enzyme is responsible for the hydrolysis of lactose into glucose and galactose.

LacY gene encodes for the enzyme permease. It is responsible for the uptake of lactose inside the bacterial cell from the environment.

LacA gene encodes for the enzyme transacetylase. The functioning of this enzyme is not yet clear but It may help in the uptake of lactose from the environment.

Lactose Operon is not 100% switched off. It is always transcribed at low levels which is called as Basal level of transcription. It is required because the Basal level encodes for the transcription of enzyme permease which is required to uptake lactose from the external environment when the concentration of lactose increases.

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