The evolution of bacterial LuxI and LuxR quorum sensing regulators

Quorum sensing is a widespread form of bacterial communication in which ind ividual cells produce and respond to specific N-acyl homoserine lactone sig nal metabolites. The different autoinducer synthases that generate these si gnals 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 phy logenetic analysis of 76 individual LuxI and LuxR homologues present in div erse members of the Gram-negative Proteobacteria. The results were consiste nt with an early origin for these regulators during the evolution of the Pr oteobacteria, with functional pairs of luxI and luxR genes possibly coevolv ing as regulatory cassettes. In many cases, specific LuxI and LuxR family m embers appeared to have been inherited horizontally. In particular, those s pecies containing multiple LuxI and/or LuxR homologues usually appeared to have obtained each individual homologue or functional pair of homologues fr om an independent source. Because multiple homologues interact to form regu latory cascades, this finding suggests that hierarchical signalling pathway s can potentially evolve by the sequential integration of pre-existing regu latory circuits acquired from diverse sources.