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

Abstract

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 Nacylhomoserine 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 BraI/R, which produces and responds to Ndodecanoyl-3-oxo-homoserine lactone (C12-3-oxo-AHL). The BraI/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 BraI/R system is similar to LasI/R of Pseudomonas aeruginosa and, as with lasI/R, there is a repressor gene, rsaL, between the braI/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. INTRODUCTIONThe genus Burkholderia in recent years has been phylogenetically well defined, consisting of species that are functionally remarkably diverse (Coenye & Vandamme, 2003). In fact, Burkholderia species have been isolated from many different environmental niches, including soil and water, and can form associations with plants, animals and humans. Phylogenetic trees, based on 16S rRNA sequence analysis, show clearly the separation of the genus Burkholderia into two major clusters (Caballero-Mellado et al., 2007), one of them mainly represented by plant, animal and human pathogenic species, including the opportunistic human ones referred to as Burkholderia cepacia complex (BCC) (Coenye & Vandamme, 2003). The BCC, a group of at least nine species, has received most attention by the scientific community since, besides being isolated from diverse environmental sources, they have also been recovered from clinical specimens. Patients with cystic fibrosis (CF) are particularly susceptible to members of the BCC, which can cause serious chronic lung infections and, together with Pseudomonas aeruginosa, represent the biggest threat to CF individuals (Coenye & LiPuma, 2003;LiPuma, 2003). Other members of the genus Burkholderia ca...

Commonalities and Differences in Regulation of N -Acyl Homoserine Lactone Quorum Sensing in the Beneficial Plant-Associated Burkholderia Species Cluster

“…In a previous study we reported the presence of an AHL-based QS system in the rice endophyte B. kururiensis designated BraI/R KUR , which produces and responds to 3-oxo-C 12 -HSL. It was determined that the BraI/R KUR system is stringently regulated by RsaL, and it is not involved in the regulation of nitrogen fixation or in other several important phenotypes (73). Importantly, BraI/R was found to be highly conserved in 20 species of the recently described cluster (73).…”

“…The method used to detect siderophores was adapted from the universal chemical assay on chrome azurol S (CAS) agar plates (66,73). The cultures were harvested, and the pellets washed and adjusted to an OD 600 of 1.…”

Biossurfactantes: propriedades anticorrosivas, antibiofilmes e antimicrobianas

“…Foi comprovado que a B. kururiensis possui o sistema HSL de quorum sensing, chamado de BraI/R, que produz e responde ao 3-oxo-C 12 -HSL e é extremamente similar ao LasI/R de P. aeruginosa (nesta é interconectado com outros sistemas regulatórios e é o regulador global envolvido na regulação de muitos genes que possuem importância na virulência e colonização). 100 Também foi relatada a existência de uma outra molécula produzida por P. aeruginosa que possui efeito antibiofilmes, o ácido graxo insaturado cis-2-decenóico, porém, seu mecanismo de ação ainda não foi elucidado. 101 Desta forma a ação dos ramnolipídios pode ser otimizada pela presença destes compostos, ajudando a inibir a formação dos biofilmes.…”

Mechanisms Controlling the Expression of the Exopolysaccharide of Burkholderia and Role in Niche Adaptation

“…AHLs are synthesized by synthase proteins and are detected by transcriptional regulators, which regulate the expression of many different target genes. The presence of well conserved QS systems have been described within Bcc species, where the system is composed of the synthase CepI and the regulator CepR (Lewenza et al, 1999); and in plant associated Burkholderia, which have a system based on the synthase BraI and the regulator BraR (Lewenza et al, 1999;Suarez-Moreno et al, 2008). These two conserved systems are not homologous and were found to be based on different AHL molecules: CepI/R system secretes and responds to C6-HSL and C8-HSL; and the BraI/R system to 3-oxo-C12-HSL and 3-oxo-C14-HSL depending in the strain (Lewenza et al, 1999;SuarezMoreno et al, 2008).…”