Cell-cell communication in bacteria is accomplished through the exchange of extracellular signalling molecules called autoinducers. This process, termed quorum sensing, allows bacterial populations to coordinate gene expression. Community cooperation probably enhances the effectiveness of processes such as bioluminescence, virulence factor expression, antibiotic production and biofilm development. Unlike other autoinducers, which are specific to a particular species of bacteria, a recently discovered autoinducer (AI-2) is produced by a large number of bacterial species. AI-2 has been proposed to serve as a 'universal' signal for inter-species communication. The chemical identity of AI-2 has, however, proved elusive. Here we present the crystal structure of an AI-2 sensor protein, LuxP, in a complex with autoinducer. The bound ligand is a furanosyl borate diester that bears no resemblance to previously characterized autoinducers. Our findings suggest that addition of naturally occurring borate to an AI-2 precursor generates active AI-2. Furthermore, they indicate a potential biological role for boron, an element required by a number of organisms but for unknown reasons.

Many bacteria control gene expression in response to cell population density, and this phenomenon is called quorum sensing. In Gram‐negative bacteria, quorum sensing typically involves the production, release and detection of acylated homoserine lactone signalling molecules called autoinducers. Vibrio harveyi, a Gram‐negative bioluminescent marine bacterium, regulates light production in response to two distinct autoinducers (AI‐1 and AI‐2). AI‐1 is a homoserine lactone. The structure of AI‐2 is not known. We have suggested previously that V. harveyi uses AI‐1 for intraspecies communication and AI‐2 for interspecies communication. Consistent with this idea, we have shown that many species of Gram‐negative and Gram‐positive bacteria produce AI‐2 and, in every case, production of AI‐2 is dependent on the function encoded by the luxS gene. We show here that LuxS is the AI‐2 synthase and that AI‐2 is produced from S‐adenosylmethionine in three enzymatic steps. The substrate for LuxS is S‐ribosylhomocysteine, which is cleaved to form two products, one of which is homocysteine, and the other is AI‐2. In this report, we also provide evidence that the biosynthetic pathway and biochemical intermediates in AI‐2 biosynthesis are identical in Escherichia coli, Salmonella typhimurium, V. harveyi, Vibrio cholerae and Enterococcus faecalis. This result suggests that, unlike quorum sensing via the family of related homoserine lactone autoinducers, AI‐2 is a unique, ‘universal’ signal that could be used by a variety of bacteria for communication among and between species.

Bacteria communicate with one another using chemical signaling molecules as words. Specifically, they release, detect, and respond to the accumulation of these molecules, which are called autoinducers. Detection of autoinducers allows bacteria to distinguish between low and high cell population density, and to control gene expression in response to changes in cell number. This process, termed quorum sensing, allows a population of bacteria to coordinately control the gene expression of the entire community. Quorum sensing confuses the distinction between prokaryotes and eukaryotes because it allows bacteria to behave as multicellular organisms, and to reap benefits that would be unattainable to them as individuals. Many bacterial behaviors are regulated by quorum sensing, including symbiosis, virulence, antibiotic production, and biofilm formation. Recent studies show that highly specific as well as universal quorum sensing languages exist which enable bacteria to communicate within and between species. Finally, both prokaryotic and eukaryotic mechanisms that interfere with bacterial quorum sensing have evolved. Specifically, the secretion of enzymes that destroy the autoinducers, and the production of autoinducer antagonists, are used by competitor bacteria and susceptible eukaryotic hosts to render quorum sensing bacteria mute and deaf, respectively. Analogous synthetic strategies are now being explored for the development of novel antimicrobial therapies.Bacteria in communities convey their presence to one another by releasing and responding to the accumulation of chemical signaling molecules called autoinducers. This process of intercellular communication, called quorum sensing, was first described in the bioluminescent marine bacterium Vibrio fischeri (Hastings and Nealson 1977;Nealson and Hastings 1979). V. fischeri lives in symbiotic associations with a number of marine animal hosts. In these partnerships, the host uses the light produced by V. fischeri for specific purposes such as attracting prey, avoiding predators, or finding a mate. In exchange for the light it provides, V. fischeri obtains a nutrient-rich environment in which to reside (Ruby 1996;Visick and McFall-Ngai 2000). A luciferase enzyme complex is responsible for light production in V. fischeri. Bioluminescence only occurs when V. fischeri is at high cell number, and this process is controlled by quorum sensing. Specifically, the production and accumulation of, and the response to, a minimum threshold concentration of an acylated homoserine lactone (HSL) autoinducer regulates density-dependent light production in V. fischeri, and enables V. fischeri to emit light only inside the specialized light organ of the host but not when free-living in the ocean. The reason for this is twofold. First, only under the nutrient-rich conditions of the light organ can V. fischeri grow to high population densities, and second, trapping of the diffusible autoinducer molecule in the light organ with the bacterial cells allows it to accumulate to a sufficient ...

A sorbitol dehydrogenase (SDH; L-iditol: NAD+ 2-oxidoreductase; EC 1.1.1.14) was isolated from the phototrophic bacterium Rhodobacter sphaeroides strain M22, a transposon mutant of R. sphaeroides Si4 with the transposon inserted in the mannitol dehydrogenase (MDH) gene. SDH was purified 470-fold to apparent homogeneity by ammonium sulfate precipitation, chromatography on Phenyl-Sepharose, Q-Sepharore and Matrex Gel Red-A, and by gel filtration on Superdex 200. The relative molecular mass (M,) of the native SDH was 61 000 as calculated from its Stokes' radius (r, = 3.5 nm) and sedimentation coefficient (S20,w = 4.235). SDS-PAGE resulted in one single band representing a polypeptide with a M, of 29 000, indicating that the native protein is a dimer. The isoelectric point of SDH was determined to be pH 4.8. The enzyme was specific for NAD+ and catalysed the oxidation of D-glucitol (sorbitol) to Dfructose, galactitol to D-tagatose and of L-iditol. The apparent K, values were NAD+, 096 mM; D-glucitol, 6 2 mM; galactitol, 1.5 mM; NADH, 013 mM; Dfructose, 160 mM; and D-tagatose, 13 mM. The pH-optimum of substrate oxidation was 11.0 and that of substrate reduction 69-7.2. It was demonstrated that SDH is expressed in the wild-type strain R. sphaeroides Si4 together with MDH during growth on D-glucitol. Forty-four amino acids of the SDH N terminus were sequenced. This sequence exhibited 45-55% identity to the N-terminal sequence of 10 enzymes belonging to the short-chain alcohol dehydrogenase family.

No abstract