Bacterial cells can take up the amino acid tryptophan (Trp) from their surroundings, or, if there is an insufficient external supply, they can synthesize tryptophan from other small molecules. The trp repressor is a bacterial gene regulatory protein that represses the transcription of genes that code for the enzymes required for the synthesis of tryptophan. Trp repressor binds to a site in the promoter of these genes only when molecules of tryptophan are bound to it.

A. Why is this a useful property of the Trp repressor?

B. What would happen to the regulation of the tryptophan biosynthesis enzymes in cells that express a mutant form of Trp repressor that (i) cannot bind to DNA or (II) binds to DNA even if no tryptophan is bound to it?

C. What would happen to scenarios (i) and (II) if the cells, in addition, produced normal Trp repressor protein from a second, normal gene?