Question

Describe the regulation of trp operon by attenuating transcription.

Answer 1

There are two types of negative feedback mechanisms regulating the tryptophan (trp) operon. One involves trp repressor and another is via attenuation mechanism. Both work to reduce the expression from trp operons when the levels of tryptophan are high. The underlying principle for transcriptional attenuation is that, in bacteria, transcription and translation proceed simultaneously.

When trp is high, transcriptional attenuation causes premature termination of transcription of mRNA encoding enzymes for tryptophan biosynthesis. This happens as follow:

Between the operator and first gene of operon there is a leader sequence that encodes a short polypeptide and also contains within it an attenuator sequence. The later is transcribed into mRNA that has potential to form hairpin structures. The polypeptide encoded by leader has two tryptophan residues. When tryptophan is scarce, ribosome stalls at trp codons of leader sequence to translate the tryptophans from it. This slowing of ribosome allows formation of anti-termination hairpin by mRNA of attenuation sequence and the transcription of whole operon continues. But when tryptophan levels are high, ribosome translates the entire leader peptide without interruption and stalls only during translation termination at the stop codon of short peptide. At this point ribosome falls off the mRNA. This allows the formation of terminator hairpin (instead of anti-terminator hairpin), making RNA polymerase detach from mRNA (encoding enzymes for tryptophan biosynthesis) and thus ends transcription of trp operon.