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 all questions

1. Draw the mel operon showing structural and regulatory genes, the regulatory region including the promoter and terminator. Describe the role of each term in your drawing.
2. Show the regulation model of the mel operon in the presence of melrose. In your figures, include structural genes, regulatory region, MelR, melrose, transcription and translation. Describe the process of how gene expression is turned ON or OFF in the presence of Melrose.
3. Show the regulation model of the mel operon in the absence of melrose. In your figures, include structural genes, regulatory region, MelR, transcription and translation. Describe the process how gene expression is turned ON or OFF in the absence of melrose.

d. Answer the following questions and provide an explanation for each answer     

i. Is the mel operon inducible or repressible?

2. Is the MelR a positive or negative regulator?

5. 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.

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

Answer 1

C) i.The mel operon is repressible because in presence of melrose transcription and translation of melrose synthesis gene stops.It binds to inactive mel repressor protein and form form repressor-melrose active dimer complex and change the confirmation of repressor and binds to the operator and prevent the binding of RNA polymerase to promoter.

ii.The MelR ia a  negative regulator when it binds to the melrose convert into active form and binds to the operon and stop biosynthesis of melrose.

a) i. In a melR1 mutant loss the ability to binds to melrose. In pressence of melrose, the melR1 is not form the complex with melrose and it is inactive form and continuously synthesized the melrose.

ii. In a melR2 mutant, MelR cannot bind to the regulatory region irrespective of the presence or absence of melrose.The melrose synthesis is not regulated by repressor and melrose. The melrose is continuously synthesized in presence and absence of melrose.

iii.In a melR3 mutant, MelR binds to the regulatory region irrespective of the presence or absence of melrose.The meltrose synthesis regulated in the absence of melrose also and no transcription and translation of melrose biosyntheszed gene in bacterial strain with these mutation.