Trp operon

Repression

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 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 threonine.

See also

• Trp repressor

References

• Morse, DE; Mosteller RD; Yanofsky C (1969). "Dynamics of synthesis, translation, and degradation of trp operon messenger RNA in E. coli.". Cold Spring Harb Symp Quant Biol. 34: 725-40. PMID 4909527.