CENSUS AND CONSENSUS IN BACTERIAL ECOSYSTEMS - THE LUXR-LUXI FAMILY OF QUORUM-SENSING TRANSCRIPTIONAL REGULATORS

The importance of accurate demographic information is reflected in the United States Constitution, Article 1, which provides for a decennial census of this country's human population. Bacteria also conduct a ce nsus of their population and do so more frequently, more efficiently, and as far we know, with little if any of the political contentiousnes s caused by human demographers. Many examples have been found of parti cular bacterial genes, operons, or regulons that are expressed prefere ntially at high cell densities. Many of these are regulated by protein s related to the LuxR and LuxI proteins of Vibrio fischeri, and by a d iffusible pheromone called an autoinducer. LuxR and LuxI and their cog nate autoinducer (3-oxohexanoyl homoserine lactone, designated VAI-1) provide an important model to describe the functions of this family of proteins. LuxR is a VAI-1 receptor and a VAI-1-dependent transcriptio nal activator, and LuxI directs the synthesis of VAI-1. VAI-1 diffuses across the bacterial envelope, and intracellular concentrations of it are therefore strongly increased by nearby VAI-1-producing bacteria. Similar systems regulate pathogenesis factors in Pseudomonas aeruginos a and Erwinia spp., as well as Ti plasmid conjugal transfer in Agrobac terium tumefaciens, and many other genes in numerous genera of gramneg ative bacteria. Genetic analyses of these systems have revealed a high degree of functional conservation, while also uncovering features tha t are unique to each.