The Quorum Sensing Regulator CinR Hierarchically Regulates Two Other Quorum Sensing Pathways in Ligand-Dependent and -Independent Fashions in Rhizobium etli

Abstract

Many rhizobial species use complex N-acyl-homoserine lactone (AHL)-based quorum sensing (QS) systems to monitor their population density and regulate their symbiotic interactions with their plant hosts. There are at least three LuxRI-type regulatory systems in Rhizobium etli CFN42: CinRI, RaiRI, and TraRI. In this study, we show that CinI, RaiI, and TraI are responsible for synthesizing all AHLs under the tested conditions. The activation of these AHL synthase genes requires their corresponding LuxR-type counterparts. We further demonstrate that CinRI is at the top of the regulatory cascade that activates RaiRI and TraRI QS systems. Moreover, we discovered that CinR possesses a specific affinity to bind cinI promoter in the absence of its cognate AHL ligand, thereby activating cinI transcription. Addition of AHLs leads to improved binding to the cinI promoter and enhanced cinI expression. Furthermore, we found that compared to the wild type, the cinR mutation displayed reduced nodule formation, and cinR, raiR, and traI mutants show significantly lower levels of nitrogen fixation activity than the wild type. These results suggest that the complex QS regulatory systems in R. etli play an important role in its symbiosis with legume hosts. IMPORTANCEMany bacteria use quorum sensing (QS) to monitor their cell densities and coordinately regulate a number of physiological functions. Rhizobia often have diverse and complex LuxR/LuxI-type quorum sensing systems that may be involved in symbiosis and N 2 fixation. In this study, we identified three LuxR/LuxI-type QS systems in Rhizobium etli CFN42: CinRI, RaiRI, and TraRI. We established a complex network of regulation between these QS components and found that these QS systems played important roles in symbiosis processes. E stablishment of symbiosis between rhizobia and their legume hosts is a complex process requiring multiple intricate signal exchanges. Many plant-associated bacteria, such as species of legume-nodulating rhizobia, use a set of diffusible N-acyl homoserine lactones (AHLs) involved in quorum sensing (QS) (1) systems to communicate with each other and optimize their interactions with plant hosts. In a typical complete LuxRI QS system, AHLs are synthesized by an LuxI-type protein and are accumulated to threshold levels as cell density increases. An LuxR-type transcriptional activator is activated by threshold levels of AHLs and induces expression of specific target genes to regulate multiple physiological functions. Quorum sensing has been implicated in various aspects of legume symbioses, including exopolysaccharide production, which is important for infection, plasmid transfer, competitiveness, nodule formation, and nitrogen fixation (2).Quorum sensing in rhizobia is very diverse, and two strains from the same species of rhizobia often do not have the same quorum sensing components (3). For example, in Sinorhizobium meliloti, SinR/SinI and ExpR in strain Rm1021 regulate exopolysaccharide production and swarming (4-6), TraR/TraI in Rm41 controls p...

The AHL Quorum-Sensing System Negatively Regulates Growth and Autolysis in Lysobacter brunescens

“…The quorum-sensing system is involved in cell-to-cell communication, regulates transcription factor activity and alters bacterial phenotypes, including biofilm, virulence, and symbiosis processes (Zheng et al, 2015; Papenfort and Bassler, 2016). During nodulation in Mesorhizobium tianshanense , the number of nodules per plant inoculated with AHL-QS mutants was significantly decreased compared to that inoculated with the wildtype strain, which indicated that the AHL quorum-sensing system was required during the symbiotic process between the bacterium and host plant Glycyrrhiza (licorice) (Cao et al, 2009).…”

Quorum Sensing in Pseudomonas savastanoi pv. savastanoi and Erwinia toletana: Role in Virulence and Interspecies Interactions in the Olive Knot

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