Question

How the trp operon is 1) not expressed when trp concentrations are high; and 2) attenuated differently in the presence and absence of trp.

Answer 1

1)When levels of tryptophan are high, attenuation causes RNA polymerase to stop prematurely when it's transcribing the trp operon. Only a short, stubby mRNA is made, one that does not encode any tryptophan biosynthesis enzymes.

2)Like regulation by the trp repressor, attenuation is a mechanism for reducing expression of the trp operon when levels of tryptophan are high. However, rather than blocking initiation of transcription, attenuation prevents completion of transcription.