The new group of non-pathogenic plant-associated nitrogen-fixing Burkholderia spp. shares a conserved quorum-sensing system, which is tightly regulated by the RsaL repressor

A novel group of nitrogen-fixing plant-associated Burkholderia species has emerged in the last few years. The purpose of this investigation was to determine if these species possess an N-acylhomoserine lactone (AHL) quorum-sensing (QS) cell-cell signalling system, and whether it is important for nitrogen fixation and other phenotypic features in Burkholderia kururiensis. It was determined that B. kururiensis, and other members of this Burkholderia species cluster, contain at least one highly conserved system, designated Bral/R, which produces and responds to N-dodecanoyl-3-oxo-homoserine lactone (C12-3-oxo-AHL). The Bral/R AHL QS is not involved in the regulation of nitrogen fixation or in several other important phenotypes, indicating that it may not be a global regulatory system. The Bral/R system is similar to Lasl/R of Pseudomonas aeruginosa and, as with lasl/R, there is a repressor gene, rsaL, between the bral/R genes. B. kururiensis normally synthesizes very low levels of C12-3-oxo-AHL, but the situation dramatically changes when RsaL is missing since an rsaL mutant displays a marked increase in AHL production. This unique stringent regulation indicates that RsaL could be an on/off switch for AHL QS in B. kururiensis and the ability to produce very high levels of AHL also questions the role of this molecule in the novel group of Burkholderia. The presence of a well-conserved and distinct AHL QS system among all the diazotrophic Burkholderia is a further indication that they are closely related, and that this system might play an important and conserved role in the lifestyle of this novel group of bacterial species.