| Abstract: | The nucleotide sequence of the luxR and luxl genes from Vibrio fischeri strain ATCC 7744 was determined. The deduced amino acid sequences of LuxR and Luxl reveal that they are proteins 250 and 193 amino acids in length with subunit molecular weights of 28,518 daltons and 21,937 daltons, respectively. The LuxR protein shows similarity to the carboxyl terminal portion of the UhpA/FixJ family of transcriptional activators and has significant overall similarity to a sub-set of that family. Several of the members of that sub-set, named the LuxR sub-family, have been shown to be activated by V. fischeri autoinducer and other N-alkyl homoserine lactones. This class of signal molecules appears to be widely utilized throughout the bacterial kingdom and promises to be a conserved mediator of coordinated regulation in bacterial populations. The LuxR protein binds to nucleotide sequences immediately adjacent to the promoter for the luxICDABEG operon and, in conjunction with N-(3- oxohexanoyl)homoserine lactone (autoinducer) constitutes a highly inducible positive regulator of lux transcription. The DNA sequences required for LuxR-dependent transcriptional activation are contained within a 20 bp region of dyad symmetry. The ability of a single amino acid substitution in LuxR to suppress two symmetric mutations in its DNA binding site (separated by 11 bp), suggests that LuxR is a multimer with a two-fold axis of symmetry. Based on similarities to other DNA-binding proteins, LuxR is believed to be a helix-turn-helix DNA-binding protein. The proposed helix-turn-helix motif for LuxR includes residues 200 to 219. Presumably, one such DNA-binding element specifically interacts with each half of the symmetric LuxR binding site. Residue 212 of LuxR, an arginine, appears to interact with the G-C base pair at position 5 of the binding site and its symmetry-related partner, a C-G base pair at position 16. The lysyl residue at position 211 may reside near the 3 and 18 positions of the LuxR binding site when LuxR is bound, but there is no evidence for specific interactions. |
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