Final answer:
Attenuation is a regulatory system that controls the expression of the trp operon in E. coli. Stem-loop structures formed within the mRNA determine whether transcription and translation will occur in the presence or absence of tryptophan. Excess tryptophan blocks RNA polymerase from transcribing the operon, while low levels of tryptophan allow transcription to proceed.
Step-by-step explanation:
In attenuation, stem-loop structures formed within the 5' end of an mRNA determine whether transcription will occur and if the mRNA will be used for translation. In the trp operon, when tryptophan is plentiful, a terminator loop forms and transcription terminates. When tryptophan levels are low, translation of a short leader peptide stalls, allowing an antiterminator loop to form, and transcription proceeds.
Excess tryptophan binds to the trp repressor and prevents RNA polymerase from transcribing the operon. This blocks the expression of the trp genes. Conversely, when tryptophan is absent, the repressor protein does not bind to the operator and the genes are transcribed.