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.

FIG. 1.

All of the differentially expressed genes detected in the microarray experiments are grouped by cluster analysis based on the similarity of their regulation (groups A to H). The genes in groups A to H are further classified into different categories based on their function (x axis). The number of genes which have similar functions and modes of regulation is indicated in the y axis. COG categories: E, amino acid transport and metabolism; H, coenzyme transport and metabolism; G, carbohydrate transport and metabolism; P, inorganic ion transport and metabolism; U, intracellular trafficking; F, nucleotide transport and metabolism; J, translation; Q, secondary metabolite biosynthesis; C, energy production and conversion; T, signal transduction mechanisms; D, cell cycle control; M, cell wall; N, cell motility; V, defense mechanisms; S, function unknown; K, transcription; L, replication; O, posttranslational modification; R, general function prediction only; X, no function.

FIG. 2.

Model for the regulation of S. meliloti gene expression by the SinR/SinI/ExpR quorum sensing system and other possible quorum sensing system(s). Continuous arrows denote relationships that can be affected by more than one way of regulation. Dotted and dashed arrows indicate unbranched relationships.