In Pseudomonas aeruginosa the LasR protein is required for activation of lasB and several other virulence genes. A diffusible signal molecule, the P. aeruginosa autoinducer (PAI), produced by the bacterial cel and released into the growth medium, is required for activity ofLasR. By cloning a lasB::lacZ fusion and a lasR gene under control of the lac promoter in Escherichia coli, we have developed a quantitative bioassay for PAI. We have used this assay to foUlow the purification of PAI from cel-free culture supernatant fluids in which P. aeruginosa or E. coli containing the P. aeruginosa gene required for autoinducer synthesis, lasI, had been grown. Chemical analyses indicated the purified material was 3-oxo-N-(tetrahydro-2-oxo-3-furanyl)dodecanamide. To confirm this assignment, the compound was synthesized and the synthetic compound was shown to have chemical and biological properties identical to those of PAI purified from culture supernatant fluids. The elucidation of the PAI structure suggests therapeutic approaches toward control of P. aeruginosa infections.

Quorum sensing is a widespread form of bacterial communication in which individual cells produce and respond to specific N-acyl homoserine lactone signal metabolites. The different autoinducer synthases that generate these signals and the receptor/activator proteins that mediate the cell's response to them constitute evolutionarily conserved families of regulatory proteins known as the LuxI and LuxR families, respectively. We have performed a phylogenetic analysis of 76 individual LuxI and LuxR homologues present in diverse members of the Gram-negative Proteobacteria. The results were consistent with an early origin for these regulators during the evolution of the Proteobacteria, with functional pairs of luxI and luxR genes possibly coevolving as regulatory cassettes. In many cases, specific LuxI and LuxR family members appeared to have been inherited horizontally. In particular, those species containing multiple LuxI and/or LuxR homologues usually appeared to have obtained each individual homologue or functional pair of homologues from an independent source. Because multiple homologues interact to form regulatory cascades, this finding suggests that hierarchical signalling pathways can potentially evolve by the sequential integration of pre-existing regulatory circuits acquired from diverse sources.

The Sym plasmid pRL1JI encodes functions for the formation of nitrogen-fixing pea root nodules by Rhizobium leguminosarum. Some of the nodulation genes are involved in recognition of chemical signals produced by the plant root, and others are required for production of chemical signals recognized by the plant. pRL1JI also contains a regulatory gene, rhiR, that is homologous to luxR, the transcriptional activator of luminescence genes in Vibrio fischeri. LuxR requires a signal compound, an autoinducer, for its activity. We have identified an R. leguminosarum autoinducer that, together with RhiR, is required to activate both the rhizosphere-expressed rhiABC operon and a growth-inhibiting function encoded by pRL1JI. This intercellular signal is an N-acylated homoserine lactone structurally related to the V. fischeri and other autoinducers. These findings indicate a new level of intercellular communication in root nodule formation.

No abstract

Autoinduction is a conserved mechanism of cell density-dependent gene regulation that occurs in a variety of gram-negative bacteria. Autoinducible luminescence in Vibrio fischeri requires a transcriptional activator, LuxR, while a LuxR homolog, LasR, activates elastase expression in Pseudomonas aeruginosa. Both LuxR and LasR require specific signal molecules, called autoinducers, for activity. We show here the activation in Escherichia coli of the V. fischeri luminescence (lux) operon by LasR and of the P. aeruginosa elastase gene (lasB) by LuxR when each is in the presence of its cognate autoinducer. Neither LuxR nor LasR showed appreciable activity with the heterologous V. fischeri or P. aeruginosa autoinducer. This supports the view that there is a direct interaction of each transcriptional activator with its proper autoinducer and suggests that there are conserved, autoinduction-related elements within the promoter regions of these genes.