The LuxR Homolog ExpR, in Combination with the Sin Quorum Sensing System, Plays a Central Role in Sinorhizobium meliloti Gene Expression

Abstract

Quorum sensing, a population density-dependent mechanism for bacterial communication and gene regulation, plays a crucial role in the symbiosis between alfalfa and its symbiont Sinorhizobium meliloti. The Sin system, one of three quorum sensing systems present in S. meliloti, controls the production of the symbiotically active exopolysaccharide EPS II. Based on DNA microarray data, the Sin system also seems to regulate a multitude of S. meliloti genes, including genes that participate in low-molecular-weight succinoglycan production, motility, and chemotaxis, as well as other cellular processes. Most of the regulation by the Sin system is dependent on the presence of the ExpR regulator, a LuxR homolog. Gene expression profiling data indicate that ExpR participates in additional cellular processes that include nitrogen fixation, metabolism, and metal transport. Based on our microarray analysis we propose a model for the regulation of gene expression by the Sin/ExpR quorum sensing system and another possible quorum sensing system(s) in S. meliloti.

Rhizobial homologs of the fatty acid transporter FadL facilitate perception of long-chain acyl-homoserine lactone signals

“…For this purpose, we determined promoter activities of several ExpRregulated genes in the WT and sinI strains and their fadL mutant derivatives grown without addition of AHLs to the cultures. Promoter activities of wgeA, wggR, SMc04171, and SMa2111, known to be activated by ExpR and AHLs (17,19), were decreased in the fadL strain ( Fig. 3A and SI Appendix, Fig.…”

“…Previous transcriptome studies detected negative regulation of fadL Sm by expR independent of sinI (17,22). We applied a fadL promoter-EGFP fusion to monitor fadL expression levels.…”

“…To test whether FadL has a role in the uptake of long-chain AHLs in S. meliloti, we analyzed the response of sinI mutant Sm2B4001 (15) (referred to as sinI strain hereinafter) to externally supplied C16:1-HSL in the presence and absence of the fadL Sm gene. Because the sinI strain is unable to synthesize AHLs, and these signal molecules are sensed by the cytosolic LuxR-type regulator ExpR (16,17), transcriptional activation of its target genes requires AHL uptake. In this setup, we determined activation of galactoglucan biosynthesis gene expression by judging culture morphology on tryptone-yeast (TY) agar and monitoring activity of the wgeA promoter.…”

“…ExpR also represses transcription of sinR at high AHL concentrations, which provokes negative feedback regulation of sinI (15). Furthermore, ExpR has a broad regulatory influence on the transcriptome, including activation of exopolysaccharide (EPS) biosynthesis and repression of motility genes (16)(17)(18). Long-chain AHL-dependent Significance Bacterial intercellular communication is crucial for developing population and community structures and for pathogenic and symbiotic interactions with eukaryotic hosts.…”

Role of Quorum Sensing in the Sinorhizobium Meliloti –Alfalfa Symbiosis

“…meliloti possesses a quorum-sensing system composed of two transcriptional regulators, SinR and ExpR, and the SinRcontrolled autoinducer synthase SinI, which is responsible for the biosynthesis of AHLs (43,44). These AHLs, in conjunction with the ExpR regulator, control a variety of downstream genes (34). Work in our laboratory has shown that the ExpR/ Sin system induces the production of both succinoglycan and EPS II and plays an important role in adjusting motility and chemotaxis in response to cell population density (34,35,42).…”