Critical regions of the Vibrio fischeri luxR protein defined by mutational analysis

Expression of Vibrio fischeri luminescence genes requires an inducer, termed autoinducer, and a positive regulatory element, the luxR gene product. A plasmid containing a tac promoter-controlled luxR was mutagenized in vitro with hydroxylamine, and luxR mutant plasmids were identified by their inability to complement a luxR deletion mutation in trans. Sixteen luxR mutant plasmids were obtained, ten of which encoded full-length but inactive luxR gene products as demonstrated by a Western immunoblot analysis. The effects of 1 of the 10 mutations could be overcome by the addition of autoinducer at a high concentration. The mutations in each of the 10 mutant plasmids that directed the synthesis of an inactive LuxR protein were identified by DNA sequencing. Of the 10 proteins encoded by the mutant luxR plasmids, 9 differed from the normally active LuxR in only a single amino acid residue. The amino acid residue substitutions in the proteins encoded by the nine mutant luxR genes clustered in two regions. One region around the middle of the polypeptide encoded by luxR was hypothesized to represent an autoinducer-binding domain, and the other region towards the carboxy terminus of the gene product was hypothesized to constitute a lux operator DNA-binding domain or a lux operator DNA recognition domain.