Autoinducer-2-Producing Protein LuxS, a Novel Salt- and Chloride-Induced Protein in the Moderately Halophilic Bacterium Halobacillus halophilus

The moderately halophilic bacterium Halobacillus halophilus carries a homologue of LuxS, a protein involved in the activated methyl cycle and the production of autoinducer-2, which mediates quorum sensing between certain species. luxS of H. halophilus is part of an operon that encodes an S-adenosylmethionine-dependent methyltransferase, a cysteine synthase, and a beta-cystathionine lyase. Expression of luxS was growth phase dependent, with maximal expression in the mid-exponential growth phase. In addition, transcription of luxS was strictly salt dependent; maximal mRNA concentrations were observed with 2.0 M NaCl in the growth medium. Chloride ions stimulated luxS transcription by a factor of three. Western blot analyses demonstrated a growth phase- and salinity-dependent production of LuxS. Moreover, cellular LuxS levels were strictly chloride dependent. Maximal accumulation of LuxS was observed at 0.5 to 1.0 M Cl(-) and depended on the salinity.