Question

A newly identified bacterial strain needs melrose as a growth factor. To synthesize melrose these bacteria use three enzymes that are encoded by the genes melA, melB and melC in the mel operon. The expression of these three genes is controlled by a regulatory protein MelR. Melrose binds to MelR, and it changes the ability of MelR to bind to the regulatory region of the mel operon. The mel genes are expressed only in the absence of melrose. The regulator MelR, only in the absence of melrose, can bind at the regulatory region of mel operon. Answer the following questions and provide an explanation for each answer

1. Is the mel operon inducible or repressible?

2. Is the MelR a positive or negative regulator?

3. Predict the expression of the mel operon in the following three melR mutants melR1, melR2 and melR3. Choose an answer as regulated or constitutive or no expression, including an explanation.

i. In a melR1 mutant, MelR has lost the ability to bind to melrose.

ii. In a melR2 mutant, MelR cannot bind to the regulatory region irrespective of the presence or absence of melrose.

iii. In a melR3 mutant, MelR binds to the regulatory region irrespective of the presence or absence of melrose.

Answer 1

1. In inducible operon, binding of molecule to the repressor prevent the binding of repressor to the operator region or regulatory region therefore keep operon ON. In repressive operon, binding of molecule to the repressor will increases the affinity of the repressor to the operator and therefore operon OFF. In the given example, binding of melrose to the MelR repressor will prevent the binding of repressor to the operator or regulatory region and therefore operon will be ON. Hence it is the example of inducible operon.

2. Binding of MelR repressor to the operator will keep operon OFF therefore it is the example of negative regulator. In positive regulator binding to the operator or regulatory region will stabilizes the binding of RNA polymerase to the promoter and induces transcription.

3.

1. No expression
Explanation: in normal case, MelR binds to the malrose and thereby preventing binding of MelR to the operator region or regulatory region and hence there is no hurdle for RNA polymerase to pass through the operator region and operon ON. But if the binding of the MelR will affect the binding to the malrose then MelR will always remain bound to the operator and RNA polymerase will not move through the operator region or regulatory region and operon will remain OFF.

2. Constitutive

Explanation: No binding of MelR indicates there will be mutation in the binding region of MelR to the regulatory region. In the absence of bind of MelR RNA polymerase will move conveniently through the regulatory region and operon will be constitutively ON

3. No expression
Explanation: As there will be binding of MelR repressor to the operator region irrespective of presence or absence of melrose, therefore RNA polymerase couldn't move through the operator or regulatory region and hence operon will be OFF.