MULTIPLE N-ACYL-L-HOMOSERINE LACTONE AUTOINDUCERS OF LUMINESCENCE IN THE MARINE SYMBIOTIC BACTERIUM VIBRIO-FISCHERI

In Vibrio fischeri, the synthesis of N-3-oxohexanoyl-L-homoserine lact one, the autoinducer for population density-responsive induction of th e luminescence operon (the lux operon, luxICDABEG), is dependent on th e autoinducer synthase gene luxI. Gene replacement mutants of V. fisch eri defective in luxI, which had been expected to produce no autoinduc er, nonetheless exhibited lux operon transcriptional activation. Mutan ts released into the medium a compound that, like N-3-oxohexanoyl-L-ho moserine lactone, activated expression of the lux system in a dose-dep endent manner and was both extractable with ethyl acetate and labile t o base. The luxI-independent compound, also like N-3-oxohexanoyl-L-hom oserine lactone, was produced by V. fischeri cells in a regulated, pop ulation density-responsive manner and required the transcriptional act ivator LuxR for activity in the lux system. The luxI-independent compo und was identified as N-octanoyl-L-homoserine lactone by coelution wit h the synthetic compound in reversed-phase high-pressure liquid chroma tography, by derivatization treatment with 2,4-dinitrophenylhydrazine, by mass spectrometry and by nuclear magnetic resonance spectroscopy. A locus, ain, necessary and sufficient for Escherichia coli to synthes ize N-octanoyl-L-homoserine lactone was cloned from the V. fischeri ge nome and found to be distinct from luxI by restriction mapping and Sou thern hybridization. N-Octanoyl-L-homoserine lactone and ain constitut e a second, novel autoinduction system for population density-responsi ve signalling and regulation of lux gene expression, and possibly othe r genes, in V. fischeri. A third V. fischeri autoinducer, N-hexanoyl-L -homoserine lactone, dependent on luxI for its synthesis, was also ide ntified. The presence of multiple chemically and genetically distinct but cross-acting autoinduction systems in V. fischeri indicates unexpe cted complexity for autoinduction as a regulatory mechanism in this ba cterium.