Attenuator (genetics)

Content sourced from Wikipedia, licensed under CC BY-SA 3.0.

Attenuation is a regulatory mechanism in bacteria that causes premature termination of transcription in some operons. Because bacteria transcribe and translate at the same time, the ribosome translating a short leader sequence can influence whether RNA polymerase finishes the RNA.

In attenuation, a leader region of the mRNA can fold into one of two structures. One form is a transcription terminator, which stops transcription early. The other form is an anti-terminator, which lets transcription continue to the rest of the operon. Which structure forms depends on signals from the cell, especially how fast the ribosome moves on the leader sequence.

The classic example is the trp operon in E. coli. The leader transcript (trpL) encodes a short peptide rich in tryptophan. If tryptophan is plentiful, the ribosome translates the leader quickly and a terminator hairpin forms, stopping transcription and preventing production of tryptophan biosynthesis enzymes. If tryptophan is scarce, the ribosome stalls on the leader peptide, the anti-terminator forms, and transcription proceeds to express the operon.

Attenuation provides fast, sensitive control and is common across many bacteria and archaea. It often links gene expression to the levels of the metabolite being produced or to other cellular signals. Some attenuators work by sequestering the Shine-Dalgarno sequence to stop translation rather than transcription, and some RNA elements resemble riboswitches that bind small molecules to change RNA structure.

This mechanism is ancient and widespread, helping repress biosynthetic genes when their products are abundant. Different systems use different signals, including uncharged tRNA levels or temperature, to influence which RNA structure forms.

Attenuation was first described by Charles Yanofsky in studies of the trp operon, showing how leader RNA structure and ribosome movement control gene expression.