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Attenuator (genetics)
Attenuation (in genetics) is a proposed mechanism of control in some bacterial operons which results in premature termination of transcription and which is based on the fact that, in bacteria, transcription and translation proceed simultaneously. Attenuation involves a provisional stop signal (attenuator), located in the DNA segment that corresponds to the leader sequence of mRNA. During attenuation, the ribosome becomes stalled (delayed) in the attenuator region in the mRNA leader. Depending on the metabolic conditions, the attenuator either stops transcription at that point or allows read-through to the structural gene part of the mRNA and synthesis of the appropriate protein.
Attenuation is a regulatory feature found throughout Archaea and Bacteria causing premature termination of transcription.〔Merino E, Yanofsky C (2005). "Transcription attenuation: a highly conserved regulatory strategy used by bacteria." ''Trends Genet'' 21:260–4.〕 Attenuators are 5'-''cis'' acting regulatory regions which fold into one of two alternative RNA structures which determine the success of transcription.〔Naville M, Gautheret, D (2009). "Transcription attenuation in bacteria: theme and variations". ''Briefings in Functional Genomics'' 9 (2):178-189.〕 The folding is modulated by a sensing mechanism producing either a Rho-independent terminator, resulting in interrupted transcription and a non-functional RNA product; or an anti-terminator structure, resulting in a functional RNA transcript. There are now many equivalent examples where the translation, not transcription, is terminated by sequestering the Shine-Dalgarno sequence (ribosomal binding site) in a hairpin-loop structure. While not meeting the previous definition of (transcriptional) attenuation, these are now considered to be variants of the same phenomena〔 and are included in this article. Attenuation is an ancient regulatory system, prevalent in many bacterial species providing fast and sensitive regulation of gene operons and is commonly used to repress genes in the presence of their own product (or a downstream metabolite).〔
==Classes of attenuators==
Attenuators may be classified according to the type of molecule which induces the change in RNA structure. It is likely that transcription-attenuation mechanisms developed early, perhaps prior to the archaea/bacteria separation and have since evolved to use a number of different sensing molecules (the tryptophan biosynthetic operon has been found to use three different mechanisms in different organisms.)〔
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