The Lux System

The Lux operon is a set of genes active in bacterial luminescence. Homologues are found in different species of luminescent bacteria, such as Vibrio fischeri, Vibrio harveyi, Vibrio (formerly Photobacterium) phosphoreum, Photobacterium leiognathi and Photorhabdus (Xenorhabdus) luminescens. Between these species there are slight differences in the order of genes. In the most studied species, V. fischeri, the system consists of two translated regions, a leftward region containing the LuxR gene and a rightward region containing the genes LuxI, C, D, A, B, E and G in this order.

Repression by H-NS

A nucleoid protein, H-NS, appears to be intricately involved in the regulation of the transcription of Lux genes. H-NS is a pleiotropic repressor protein that has been shown to bind predominantly to curved DNA caused by A-T rich regions. The H-NS protein consist of a dimerization (or multimerization) and a DNA-binding domain. Its binding to DNA appears to be DNA shape- rather than sequence-specific. The protein has also been implicated in the repression of foreign genes acquired by horizontal transfer and synthetic genes. Certain h-ns mutants have been shown to exhibit much higher levels of gene expression and bioluminescence. Several sites within the Lux system have been described as binding sites for H-NS. Such binding sites exist in both the leftward and the rightward promoter regions, within the coding sequence of LuxI, the intergenic region and start of LuxC as well as the intergenic region and start of LuxA.

Regulation by LuxR and LuxI

LuxR and LuxI are genes inolved in the quorum sensing mechanism of Vibrio fischeri. LuxI codes for an enzyme catalysing the synthesis of AHL. This compound is diffusible and acts as a signal between different cells of the population. In nature, V. fischeri uses quorum sensing to assess the size of the symbiont colony within their host organism. At the right stage of the colony life-cycle they activate bioluminescence. AHL binds to the protein product of LuxR changing its 3-dimensional shape. The C-terminal domain of the activated LuxR then interacts