Attenuator (genetics)

The attenuator plays an important regulatory role in prokaryotic cells because of the absence of the nucleus in prokaryotic organisms. The attenuator refers to a specific regulatory sequence that, when transcribed into RNA, forms hairpin structures to stop translation when certain conditions are not met.

An example is the trp gene in prokaryotic bacteria. When there is a high level of tryptophan in the region, the bacterium may not want to synthesize more because it wants to save energy. When the RNA polymerase binds and transcribes the trp gene, the ribosome will start translating. (This differs from eukaryotic cells, where RNA must exit the nucleus before translation starts.) The attenuator sequence, which follows the promoter and operator sequence, contains four domains, where domain 3 can pair with domain 2 or domain 4.

The attenuator sequence at domain 1 contains instruction for peptide synthesis that requires tryptophans. A high level of tryptophan will permit ribosomes to translate the attenuator sequence domains 1 and 2, allowing domains 3 and 4 to form a hairpin structure, which results in termination of transcription of the trp operon. Since the protein coding genes are not transcribed, no tryptophan is synthesised.

In contrast, a low level of tryptophan means that the ribosome will stall at domain 1, causing the domains 2 and 3 to form a different hairpin structure that does not signal termination of transcription. Therefore the rest of the operon will be transcribed and translated, so that tryptophan can be produced. Thus, domain 4 is an attenuator. Without domain 4, translation can continue regardless of the level of tryptophan. 

Attenuation (Genexpression)