A bacterial operon is responsible for the production of biosynthetic enzymes needed to make the hypothetical amino acid tisophane (tis). The operon is regulated by a separate gene, R. The deletion of R causes the loss of enzyme synthesis. In the wild-type condition, when tis is present, no enzymes are made; in the absence of tis, the enzymes are made. Mutations in the operator gene (O−) result in repression regardless of the presence of tis.

Required:
Is the operon under positive or negative control? Propose a model for:

a. repression of the genes in the presence of tis in wild-type cells.
b. the mutations.