Trp operon

Trp operon is an anabolic operon which promotes the production of tryptophan in the absence of tryptophan in the environment. This is regulated both by repression and by attenuation.

Repression
The repressor for the trp operon is produced upstream by the trpR gene, which is continually expressed. It creates monomers, which associate into tetramers. When tryptophan is present, it binds to the tryptophan repressor tetramers, and causes a change in conformation, which allows the repressor to bind the operator, which prevents RNA polymerase from binding or transcribing the operon, so tryptophan is not produced. When tryptophan is not present, the repressor cannot bind the operator, so transcription can occur. This is therefore a negative feedback mechanism.

Attenuation
Because repression of this operon is still "leaky," another system of controlling expression is also needed: Attenuation. At the beginning of the transcribed genes of the trp operon is a leader sequence, which codes for a very short polypeptide. Near the end of this sequence, two tryptophans are coded for next to each other. Because tryptophan is a fairly uncommon amino acid,this is highly unusual. Since in prokaryotes the ribosomes begin translating the mRNA as soon as the RNA polymerase has moved farther down the DNA sequence, upstream translation occurs simultaneously with transcription of downstream genes. So, as soon as the polymerase has created the mRNA for the leader sequence, it is being translated. When the ribosome reaches the double-trp codons, if enough trp is present, the ribosome will not be delayed, and will continue translating until it reaches the stop codon and falls off the leader transcript. A hairpin will then form in the mRNA transcript (remember, still attached to RNA polymerase on other end) between regions 1-2, and 3-4, which destabilizes the RNA polymerase and halts transcription of the rest of the operon, thus preventing production of trp. On the other hand, if there is little or no trp available, the ribosome will be delayed or stopped on the double-trp, and a hairpin will form between regions 2-3 of the mRNA instead. This does not destabilize the polymerase, so transcription and translation occur. Similar mechanism regulates the synthesis of histidine, phenylalanine and threonine.