Cell-to-cell communication in bacteria is mediated by quorum-sensing systems (QSS) that produce chemical signal molecules called autoinducers (AI).In particular, LuxS/AI-2-dependent QSS has been proposed to act as a universal lexicon that mediates intra-and interspecific bacterial behavior. Here we report that the model organism Bacillus subtilis operates a luxS-dependent QSS that regulates its morphogenesis and social behavior. We demonstrated that B. subtilis luxS is a growth-phase-regulated gene that produces active AI-2 able to mediate the interspecific activation of light production in Vibrio harveyi. We demonstrated that in B. subtilis, luxS expression was under the control of a novel AI-2-dependent negative regulatory feedback loop that indicated an important role for AI-2 as a signaling molecule. Even though luxS did not affect spore development, AI-2 production was negatively regulated by the master regulatory proteins of pluricellular behavior, SinR and Spo0A. Interestingly, wild B. subtilis cells, from the undomesticated and probiotic B. subtilis natto strain, required the LuxS-dependent QSS to form robust and differentiated biofilms and also to swarm on solid surfaces. Furthermore, LuxS activity was required for the formation of sophisticated aerial colonies that behaved as giant fruiting bodies where AI-2 production and spore morphogenesis were spatially regulated at different sites of the developing colony. We proposed that LuxS/AI-2 constitutes a novel form of quorumsensing regulation where AI-2 behaves as a morphogen-like molecule that coordinates the social and pluricellular behavior of B. subtilis.Bacteria not only behave as self-sufficient individuals but also act as communities capable of cell-cell communication (7,15,20,42,53). This social interaction leads to the coordination of communitarian activities that resemble, in their complexity, the behaviors observed in multicellular organisms (20,42,49,53,57). This microbial phenomenon is known as quorum sensing, a process by which bacteria monitor their cell population density by measuring the concentration of small secreted signal molecules called autoinducers (57). Even though a vast number of diverse quorum-sensing systems exist (41, 46, 57), they can be divided into two established paradigms that regulate the intraspecific behavior in many bacteria: (i) LuxI/LuxR-type quorum-sensing systems in gram-negative bacteria responsible for the production of N-acyl-L-homoserine lactone autoinducers or type I autoinducers and (ii) oligopeptide/two component-type quorum-sensing circuits in gram-positive bacteria responsible for the production of autoinducer peptides (4,17,26,32,44).However, in recent years, a considerable amount of information has been gained on the existence of another type of quorum-sensing system that not only seems to participate in intraspecific bacterial behavior but also seems to regulate the interspecific interactions among bacteria of different genera. This system involves the production of autoinducer-2 (AI-2) signal molecul...

Lipoic acid is an essential cofactor required for the function of key metabolic pathways in most organisms. We report the characterization of a Bacillus subtilis mutant obtained by disruption of the lipA (yutB) gene, which encodes lipoyl synthase (LipA), the enzyme that catalyzes the final step in the de novo biosynthesis of this cofactor. The function of lipA was inferred from the results of genetic and physiological experiments, and this study investigated its role in B. subtilis fatty acid metabolism. Interrupting lipoate-dependent reactions strongly inhibits growth in minimal medium, impairing the generation of branched-chain fatty acids and leading to accumulation of copious amounts of straight-chain saturated fatty acids in B. subtilis membranes. Although depletion of LipA induces the expression of the ⌬5 desaturase, controlled by a two-component system that senses changes in membrane properties, the synthesis of unsaturated fatty acids is insufficient to support growth in the absence of precursors for branched-chain fatty acids. However, unsaturated fatty acids generated by deregulated overexpression of the ⌬5 desaturase functionally replaces lipoic acid-dependent synthesis of branched-chain fatty acids. Furthermore, we show that the cold-sensitive phenotype of a B. subtilis strain deficient in ⌬5 desaturase is suppressed by isoleucine only if LipA is present.Lipoic acid (LA; 6,8-thioctic acid or 1,2-dithiolane-3-pentanoic acid) is a sulfur-containing cofactor required for the function of several key enzymes involved in oxidative and single-carbon metabolism, including pyruvate dehydrogenase, 2-oxoglutarate dehydrogenase, branched-chain 2-oxoacid dehydrogenase (BCKADH), acetoin dehydrogenase, and the glycine cleavage system (10). Lipoate-requiring complexes typically contain three protein subunits, E1, E2, and E3. LA is linked through an amide bond to lysine residues in the E2 subunits (42) and acts as a swinging arm, transferring covalently attached reaction intermediates among the active sites of the enzyme complexes (40).Although the general role of LA as a bound cofactor has been known for decades, the mechanisms by which LA is synthesized and becomes linked to its cognate proteins in different organisms continue to be elucidated. The reactions whereby LA-modified proteins are produced are best understood in Escherichia coli. In this organism, lipoylation is mediated by two separate enzymes, lipoyl protein ligase A (LplA) and octanoyl-acyl carrier protein-protein transferase (LipB) (30, 31). While LplA uses exogenous LA, LipB transfers endogenous octanoic acid to the target proteins (19). These octanoylated domains are then converted into lipoylated derivatives by the S-adenosyl-L-methionine-dependent enzyme lipoyl synthase (LipA), which catalyzes the insertion of sulfur atoms into the carbon-6 and -8 positions of the corresponding fatty acids (29). This process bypasses the requirement for an exogenous supply of LA.In contrast to the wealth of knowledge available on LA synthesis and utilization in E....