Natural bioluminescent bacteria

Bioluminescence is a trait found in a number of marine bacteria. Some strains use their ability to emit light to form symbiotic relationships. A number of deep sea fish and squids have specialised light organs which harbour populations of bacteria which help their hosts by emitting light. One such example is the partnership between the Hawaiian Bobtail squid (Euprymna scolopes) and the bacterium Vibrio fischeri. At night squid hunt high in the water column, attacking from above. Vibrio Fischeri is usually found in symbiosis with the Hawaiian Bobtail Squid. The squid uses the light produced by its symbionts to hide its own shadow it casts when hunting in top waters in clear moonlit nights. An elaborate light sensing and shutter system adjusts the light output to the light that falls on the squid back. Other bioluminescent bacterial species can differ markedly in their lifestyles. Vibrio Harveyi is a free living marine bacterium, while Xenorhabdus luminescens is a symbiont of terrestrial nematodes.

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. LuxA and LuxB encode the two subunits of the bacterial luciferase, while the products of LuxC, LuxD and LuxE synthesize the substrate for the light emitting reaction, tetradecanal. The exact function of LuxG is unknown, and it appears to be non-essential for light emission, but its presence increases light output. Due to the specific codon usage in the Lux operon, LuxA and LuxB are translated at a five times higher level than C, D, E and G.

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 a specific N-acyl homoserine lactone (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 colony density 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 with H-NS proteins bound to curved DNA-regions in the Lux operon. Such regions are especially prominent in the promoter regions of LuxI and LuxR, causing AHL to induce its own synthesis. Repression also occurs at the LuxCDABEG promoter as well as within the coding sequence of LuxC, LuxA and LuxB. While the natural quorum-controlled mechanism relieves this repression, it remains a problem if the lux operon is placed under a different promoter in a H-NS wild type strain.