Afictional 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.