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QS relies on the the synthesis, accumulation and subsequent sensing of a signal molecule. In V. fischeri, the signal (3-oxo-C6-HSL, an N-acyl homoserine lactone or AHL) is synthesised by the protein LuxI and sensed by the protein LuxR. At low cell densities (i.e. when only a small number of bacteria are present) the signal is produced by the bacteria at a low level. The AHL diffuses out of the bacterial cells and into the surrounding environment. As the cell density increases (i.e. more bacteria are present) the signal accumulates in the area surrounding the bacteria. When the signal reaches a critical threshold concentration, it is able to interact with the LuxR protein. The LuxR/AHL complex binds to a region of DNA called the lux box causing the luminescence genes to switch on. In addition, the LuxR/AHL complex also causes the AHL (via LuxI) to be produced at a higher level. Thus the AHL is said to autoinduce its own synthesis (see model for more details). V. fischeri exist at low cell densities when free living and at high cell densities when colonising the light organ, QS can therefore explain why the bacteria are dark when free living and light when in the the light organ. The bacteria are effectively communicating, as a single bacterium is able to detect and respond to signals produced by the surrounding bacteria. Bacteria sense their cell density by measuring the amount of signal present. A large number of Gram negative bacteria have been found to have AHL-based QS systems similar to V. fischeri. For examples of these see the table of organisms page. |