A fictional bacterial operon is responsible for the production of the biosynthet

Question

A fictional bacterial operon is responsible for the production of the biosynthetic enzymes needed to make the theoretical amino acid Tisophane (Tis). In the wild? type condition, when Tis is present, no enzymes are made. In the absence of Tis, the enzymes are made. The operon is regulated by a separate gene, R, deletion of which causes loss of enzyme production, regardless of whether Tis is present.  A second class of regulatory mutations has been identified and these map to a region (the RB site) near the promoter of the Tis operon; these so?called RB? mutations also block production of the enzymes, regardless of the availability of Tis.  

a. Is the operon under positive or negative control? Justify your answer.

b. Propose a model for regulation of the operon which explains the lack of enzyme production in the presence of Tis in wildtype cells, the role of the R protein and the effects of the RB? mutations.

Explanation / Answer

Ans. a.) The mentioned operon is in positive control due to the fact that loss of regulatory R gene product results in the loss of enzyme synthesis. The regulatory R gene generally enhances the transcription, but its activity is reduced in the presence of Tisophane (Tis). Whereas in the absence of (Tis), regulatory R protein positively controls the transcription.

b.) The proposed model for regulation of the operon would contain activator, repressor, co-repressor and a regulatory region, This model displays that Tis / B complex inhibits the transcription of Tis enzymes by binding to the B region of RB operator. The presence of R protein reduces the affinity of Tis / B complex to B region of RB. R protein has the ability to bind to the R region on RB and eventually alters the conformation of B region. This alteration of B region results in reduced affinity of Tis / B complex. Thus, in the absence of R gene, Tis / B complex bind firmly to the RB region and inhibit the transcription.
When R site is mutated at RB site, R protein is not able to bind which does not alters the Tis / B complex and thus reduced the transcription.

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