| Summary: | The LuxS enzyme, an S-ribosyl-homocysteine lyase, catalyzes the production of the signal precursor for autoinducer-2 mediated quorum sensing (QS-2) in <em>Vibrio</em>. Its widespread occurrence among bacteria is often considered the evidence for a universal language for interspecies communication. Presence of the <em>luxS</em> gene and production of the autoinducer-2 (AI-2) signal have repeatedly been the only evidences presented to assign a functional QS-2 to the most diverse species. In fact, LuxS has a primary metabolic role as part of the activated methyl cycle. In this review we have analyzed the distribution of QS-2 related genes in <em>Enterobacteriaceae</em> by moving the focus of the investigation from AI-2 production to the detection of potential AI-2 receptors. The latter are common in pathogens or endosymbionts of animals, but were also found in a limited number of <em>Enterobacteriaceae</em> of the genera <em>Enterobacter</em>, <em>Klebsiella</em>, and<em> Pantoea</em> that live in close association with plants or fungi. Although a precise function of QS-2 in these species has not been identified, they all show an endophytic or endosymbiontic lifestyle that suggests a role of type-2 quorum sensing in the adaptation to closed ecosystems.
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