Which of the following statements about the mechanism of attenuation is true? a. when tryptophan levels are low, transcription of the leader sequence proceeds, but mrna synthesis is terminated approximately 140 nucleotides along the transcript. b. when tryptophan levels are low, transcription is initiated and continues through the leader region into the structural genes, resulting in the production of a polycistronic mrna and, ultimately, the enzymes required for the biosynthesis of tryptophan. c. the levels of tryptophan in the environment affect only repression of the trp operon and do not affect attenuation. d. when tryptophan is present at high concentrations, the repressor binds to the operator and transcription is initiated and not terminated. e. as a result, the enzymes required for the biosynthesis of tryptophan are translated.