Question

Draw the lac operon promoter region with all its regulators (and polymerase if appropriate) in the PRESENCE OF LACTOSE and the ABSENCE OF GLUCOSE in the growth media in bacteria with a mutation in the lac repressor which prevents it from binding lactose and a mutation in the CAP that prevents it from binding cAMP. Indicate the level of transcription from the lac operon under those conditions (no expression, low expression, or high expression).

Answer 1

Lac operon contains three structural genes under the control of a common regulatory circuit. It is an example of inducible negative regulation. It is a catabolic operon. The structural genes code for enzymes required for the catabolism of lactose.

Lac Z = Beta-galactosidase = required for the hydrolysis of lactose

Lac Y = Permease = required for the transport of lactose

Lac A = Transacetylase

When lactose is absent, Lac I binds to the operator and prevents the recruitment of RNA polymerase to the promoter.

When lactose is available, it binds to Lac I and prevents its binding to the operator. Now, the promoter is free to be bound by the RNA polymerase and transcription of the structural genes takes place.

If there is a mutation in the lac repressor that makes it unable to bind to the operator, the operon becomes constitutively active irrespective of the presence or absence of lactose.

cAMP binds to CRP and facilitates the recruitment of RNA polymerase to the promoter in the absence of glucose. If CRP is mutated, lac operon is turned off.