Bacterial cells can take up the amino acid tryptophan from their surroundings, or, if the external supply is insufficient, they can synthesize tryptophan from small molecules in the cell. When external supplies of tryptophan are plentiful, the cells suppress transcription of the trp operon, which encodes the tryptophan biosynthetic enzymes. When external supplies of tryptophan are not plentiful, the cells express the trp operon.
The trp operon repressor protein inhibits transcription of the genes in the trp operon. Upon binding tryptophan, the tryptophan repressor binds to a site in the promoter of the operon and represses transcription.
A. (2 pts) Why is tryptophan-dependent binding to the operon a useful property for the tryptophan repressor?
B. (4 pts) How would regulation of transcription of tryptophan biosynthetic enzymes be affected in cells that express a mutant form of the tryptophan repressor that (i) cannot bind to the DNA or (ii) bind to DNA even when no tryptophan is bound to it? Also address how tryptophan synthesis would be affected and how that would affect the cell. Please bullet point your response (i) and (ii) for each mutant for ease of grading.