Solutions For An Introduction to Genetic Analysis Chapter 11 Problem 5P

Mutations that inactivate the trpA gene will block the synthesis of tryptophan in the cells, resulting in trp‾ auxotrophs.

In trp operon with wild type genotype, when level of tryptophan is low the ribosome pauses in the region 1 of the mRNA UTR. This pause allows base pairing of the regions 2 and 3 to form the anti-terminator hairpin, which permits transcription to continue and thus, trp structural genes are expressed.

If due to the mutation, tryptophan codons are replaced by alanine codons, then this normal pausing of ribosome in the region 1 during low levels of tryptophan would not occur. This would result in formation of the termination hairpin by base pairing of region 1 with the region 2 and base pairing of the regions 3 and 4, stopping transcription.

Furthermore, stalling of ribosome would occur when alanine is low, thus expression of trp structural genes would occur when alanine is present in low levels.