Attenuation is a very smart way of gene regulation which is known from bacterial cells using two alternatice hairpin structures. For example, E. coli only needs very little amounts of Tryptophan in its metabolism, so the amino-acyl-Synthetase for Tryptophan is only rarely synthesized. So the trp-operon contains an attenuator before the actual enzymes. If Tryptophan is absent, the rare tRNA loaded with Tryptophan will not be available at once, so the Ribosome is stalled. Sterics do not allow the formation of a certain stemloop with the ribosome attached. If there is Tryptophan available and many tRNATrp float through the cell, the ribosome can just continue, a stem loop is formed and the ribosome falls off: The transcription of the following trp-operon is terminated.
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