IPTG (isopropyl thio-ß-D-galactoside)
Figure 6.11. Isopropylthio-p-D-galactoside (IPTG), which can bind to the lac repressor protein but which is not metabolized.
Quorum Sensing: Squids That Glow in the Dark
The bacterium Vibrio fischeri lives free in seawater but is also found at high densities in the light-emitting organs of the nocturnal squid Euprymna scolopes, where it synthesizes an enzyme called luciferase that generates light. This phenomenon is called bioluminescence. When living free in seawater V. fischeri synthesizes almost no luciferase; indeed, there would be little point in doing so because the light emitted by a single bacterium would be too dim for anything to see. V. fischeri only begins making lots of luciferase when the density of bacteria is high—just as it is in the squid light organs. The word quorum is defined in Webster's Dictionary as "the number of... members of a body that when duly assembled is legally competent to transact business." Thus quorum sensing is a good description of V fischeri's behavior. How does it work?
Luciferase is a product of the lux A and lux B genes in a bacterial operon called the lux operon. A region in the lux operon promoter binds a transcription factor, LuxR, which is only active when it has bound a small, uncharged molecule called N-acyl-HSL (also called VAI for V. fischeri autoinducer). N-acyl-HSL in turn is made by an enzyme called N-acyl-HSL synthase (or VAI synthase) that is encoded by the gene lux I, which is part of the lux operon. In free-living V. fischeri the lux operon is transcribed at a low level. Small amounts of N-acyl-HSL are made, which immediately leak out of the cell into the open sea without binding to luxR. When the bacterium is concentrated in the squid's light organs, then some N-acyl-HSL binds to LuxR, increasing transcription of the lux operon. This makes more luciferase—but it also makes more N-acyl-HSL synthase. The concentration of N-acyl-HSL therefore rises—so transcription of the lux operon increases further. This means that the genes for N-acyl-HSL synthase, luciferase, and the enzymes that produce the substrate for luciferase are now transcribed at a high rate. This autoinduction of the lux operon by N-acyl-HSL is a form of positive feedback (page 303). When luciferase carries out its reaction, the squid will luminesce at the intensity of moonlight so that as it glides at night over the coral reefs of Hawaii it does not cast a dark shadow; thus its prey are less likely to notice its presence.
lux R operator promoter lux I lux A lux B
inactive transcription factor
RNA polymerase inactive transcription factor
RNA polymerase transcription
synthase Luciferase light
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