Solutions For An Introduction to Genetic Analysis Chapter 11 Problem 22P

(b) i. trpA is not synthesized in the presence of tryptophan; it is synthesized in the absence of tryptophan.

ii. trpA is not synthesized in the presence of tryptophan; it is synthesized in the absence of tryptophan. The second operon contains a trpA^- mutation and is unable to make any active tryptophan synthetase enzyme. However, this operon contains a wild-type trpR gene, which encodes a functional repressor molecule. Since this gene product is a diffusible molecule that is trans-acting, it will act on the other deoxyribonucleic acid (DNA) molecule containing the mutant trpR^- gene and bind to the operator on that molecule. Thus, trpA gene expression will be repressed when tryptophan binds the repressor, causing the repressor to bind the operator. In the absence of tryptophan, the repressor will not bind to either operator, and transcription will proceed.

iii. trpA is synthesized both in the presence and in the absence of tryptophan. Because the second operon conatins a mutant trpA^- gene, no functional tryptophan synthetase enzyme will be made from this DNA molecule. The first operon contains a wild-type trpA gene and will be responsible for the intracellular supply of tryptophan synthetase and ultimately, tryptophan. However, this operon contains a mutant (trpO^-) operator region that cannot be bound by the repressor molecules. Because the operator region is cis-acting and does not encode a diffusible gene product, the wild-type trpO gene on the other DNA molecule cannot substitute for the mutant operator. Therefore, the trpA gene will always be transcribed continuously regardless of the levels of tryptophan in the cell.