Question

Regulation of the lac operon is under both negative and positive control. Describe how these two transcriptional control mechanisms regulate expression of the lac operon in the presence of glucose and/or lactose.

Answer 1

Regulation of lac operon

The promoter's activity which control the lac operon expression is regulate by 2 different protein. These two protein play two different functions

Protein 1. from transcribing prevent the RNA polymerase (negative control)

Protein 2. bind the RNA polymerase to the promoter is enchanced by the other protein (positive control).

Negative control

lac repressor is the protein which inhibit the lac operon transcription. This is a tetramer wwhich have 4 identical subunit. The lacI gene is encode the  lac repressor. The lacI gene have own promotor and located in the  lac operon's upstream. Here lacI gene's expression is not regulated. Those expression is not regulated are known as constitutive genes. The lacI repressor are continuously synthesized at very low level.

At the operator the expression of lac operon isblocked by binding with the DNA was done by lac repressor in the absence of lactose. And the operator is in the downstream of promotor and upstream of inittiation site of transcription. A specific nucleotide that was in operator, which block the initiation of transcription by the tight binding of the repressor.  

The lac repressor have a great affinity for lactose. If the lactose present in the small amount the lac repressor bind to it, this result in the gene expression of operon by the dissociation from the DNA operator.

Inducers: The substrate which causes the dissociation of repressor from DNA operator.

Inducible genes: The gene are regulated by the repressor that dissociate from the DNA operator.  

Positive control

Only a very low expression of the lac operon can induced by the lactose beacuse the subjection of lac operon to catabolite repressor or by the growth that occurs in the presence of growth. The glucose prevent the lac operon expression becacuse they are easily metabolised and fuel source above the lactose.

To form a open complex and ability to bind with the RNA polymerase is considered as the strength of promoter. The promoter is weak for the lac operon and makes the lac operon poor transcription on induction.

CAP : Catabolite Activator Protein ( binding site: promoter upstream)

Due to the binding of CAP protein the binding of RNA polmerase is effectively take place by the disortind of DNA. As result the lac operon transcription is highly enhanced. First binds to cyclic AMP for the binding with CAP. The cyclic AMP / cAMP (second messenger) is synthesized by enzyme Adenylate cyclase from the ATP.

The cAMP is low and the transcription initiation from lac operon is very low due to the presence of glucose. The concentration the cAMP increases when the glucose level decreases which activate the CAP to bind to CAP site which stimulate trascription. This cAMP-CAP comples known as positive regulator.