LuxS and Autoinducer-2: Their Contribution to Quorum Sensing and Metabolism in Bacteria
Introduction
It has been known for some time that the marine bacterium Vibrio harveyi regulates bioluminescence in concert with cell density via the production of two small diffusible signal molecules termed “autoinducers.” The structure of the first V. harveyi autoinducer was elucidated in 1989 as N-(3-hydroxybutanoyl)-l-homoserine lactone (Cao 1989, Cao 1993), but the nature of the second (autoinducer-2: AI-2), remained elusive. In 1999, a gene termed luxS, required for the production of AI-2, was identified (Surette et al., 1999), and putative homologues of this gene were found in diverse bacterial genera. Since then, there has been an exponential increase in the number of luxS-related publications. Clearly, the hope of discovering AI-2–based signaling in organisms lacking one of the well-established cell-to-cell communication systems has motivated groups with little previous experience to enter the quorum sensing field. Recent efforts culminated in the discovery of the biochemical conversion catalyzed by the LuxS protein, the crystallization of not fewer than four different LuxS homologues, and the remarkable finding that AI-2 appears to be a furanosyl borate diester. AI-2 production has now been demonstrated for a large number of bacterial species, leading to the hypothesis that AI-2 is employed for interspecies communication. However, LuxS plays an important role in bacterial metabolism, contributing to the recycling of SAM, which raises the question whether the LuxS⧸AI-2 system really constitutes a signaling system in all of the organisms in which it is found. Here, we examine critically the current knowledge base and discuss future directions for LuxS research.
Section snippets
Overview of Bacterial Cell-to-Cell Communication
Bacteria are known to release a large variety of small molecules, including siderophores, secondary metabolites, and metabolic end products. They also produce and respond to diffusible compounds best described as signal molecules (sometimes termed autoinducers or pheromones), which are known to play a crucial role in bacterial cell-to-cell communication. It is believed that in many instances these molecules are used to determine the size (or density) of bacterial populations: The signal
The AI-2 Bioassay
At present, there is no biochemical assay for the direct detection and quantification of AI-2. Instead, AI-2 activity present in culture supernatants or formed during in vitro reactions is detected with a specific reporter strain first used by Bassler 1994, Bassler 1997 to screen the supernatants of various bacterial cultures for signal molecule activity. In common with an earlier assay described by Greenberg et al. (1979), it assesses the ability of cell-free culture supernatants to induce the
AI-2: A Furanone Derivative
The possibility of performing the LuxS reaction in vitro has provided the means to generate large amounts of comparatively pure AI-2 which can be used to elucidate its chemical properties and its structure. The known components of the in vitro reaction include SRH, adenine, and homocysteine. These compounds have been detected and quantified by conventional methods (Schauder 2001, Winzer 2002a). For instance, as a standard assay for LuxS activity, the amount of homocysteine liberated from SRH
LuxS Is Identical to the Ribosylhomocysteine Cleavage Enzyme
The enzymatic cleavage of SRH into homocysteine and a carbohydrate-like compound was first described for E. coli in the 1960s, and the enzyme responsible was termed the SRH-cleavage enzyme (Duerre 1966, Miller 1968). Although absent from eukaryotes, the reaction was later confirmed for other bacteria (Walker 1975, Shimizu 1984). Duerre et al. (1971) identified the carbohydrate-like compound as DPD, a known precursor in MHF formation (Nedvidek et al., 1992). MHF was shown to be present in
The Metabolic Role of LuxS
In most publications to date, LuxS is assumed to be dedicated to AI-2 production and cell-to-cell communication, and further physiological functions have rarely been taken into consideration. Only a few reports have pointed out that LuxS plays a role in a recycling pathway linked to methionine metabolism, which may, in fact, be its most important function in many organisms (Winans 2002, Winzer 2002a, Winzer 2002b). Indeed, when Miller and Duerre (1968) first analyzed this enzyme in E. coli more
The Location of luxS and pfs in Bacterial Genomes
It has been argued that the Pfs⧸LuxS pathway is primarily used for the recycling of SAH, thus enabling the cells to make economic use of methionine as a methyl donor (Winzer et al., 2002a). Interestingly, this link with methionine metabolism is also reflected in the organization of the pfs and luxS genes on some bacterial chromosomes (Fig. 7). Here, luxS or pfs are often located next to genes involved in methionine and cysteine de novo biosynthesis (cysteine provides the sulfur atom found in
Regulation of luxS Expression and AI-2 Production
The accumulation of AI-2 during growth and, in some cases, its subsequent removal from the culture fluid has been well documented. However, little is known about the regulation of the Pfs⧸LuxS pathway itself and the parameters which govern export and uptake of AI-2. Although metabolic flux through the activated methyl cycle is an absolute requirement for AI-2 production, it is not necessarily always linked with the exclusion of AI-2 from the cell. Therefore, the regulation of pfs and luxS
Mechanisms of Bacterial Cell-to-Cell Communication
Section II describes the variety of bacterial cell–cell communication (quorum-sensing) mechanisms discovered to date. Although LuxS clearly has an important role within the activated methyl cycle (as discussed in detail in preceding sections), the molecule it generates (AI-2) appears to function as a diffusible signal molecule in some bacteria. Indeed, it was in this context that it was first discoved in V. harveyi (Bassler et al., 1994). As such, AI-2-dependent quorum sensing would be unique
The Role of AI-2 in Interspecies Communication and Alternative Functions
Given that AI-2 is produced by so many different bacteria, it is perhaps not surprising that the molecule has given rise to much speculation about its function (Bassler 1999, Fuqua 1998, Coulthurst 2002, Winans 2002, Winzer 2002a, Winzer 2002b). Building on suggestions first unveiled by Greenberg et al. (1979) regarding the involvement of V. harveyi in bacterial cross-talk, the idea that AI-2 may serve as a signal in interspecies communication has been put forward by Bonnie Bassler's group and
Acknowledgements
We thank the following institutions for the invaluable resources containing details of finished and unfinished genomes: The Institute of Genomic Research (TIGR), USA; The Sanger Centre, UK; National Center for Biotechnology Information (NCBI), USA; Baylor College of Medicine, Texas, USA; DOE Joint Genome Institute, USA; Forsyth Dental Center, Boston, USA; Genome Sequencing Center, Washington University in St Louis, USA; National Yang Ming University Genome Research Centre, Taiwan; University of
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