Transcriptional Regulator LsrB of Sinorhizobium meliloti Positively Regulates the Expression of Genes Involved in Lipopolysaccharide Biosynthesis

Abstract

c Rhizobia induce nitrogen-fixing nodules on host legumes, which is important in agriculture and ecology. Lipopolysaccharide (LPS) produced by rhizobia is required for infection or bacteroid survival in host cells. Genes required for LPS biosynthesis have been identified in several Rhizobium species. However, the regulation of their expression is not well understood. Here, Sinorhizobium meliloti LsrB, a member of the LysR family of transcriptional regulators, was found to be involved in LPS biosynthesis by positively regulating the expression of the lrp3-lpsCDE operon. An lsrB in-frame deletion mutant displayed growth deficiency, sensitivity to the detergent sodium dodecyl sulfate, and acidic pH compared to the parent strain. This mutant produced slightly less LPS due to lower expression of the lrp3 operon. Analysis of the transcriptional start sites of the lrp3 and lpsCDE gene suggested that they constitute one operon. The expression of lsrB was positively autoregulated. The promoter region of lrp3 was specifically precipitated by anti-LsrB antibodies in vivo. The promoter DNA fragment containing TN11A motifs was bound by the purified LsrB protein in vitro. These new findings suggest that S. meliloti LsrB is associated with LPS biosynthesis, which is required for symbiotic nitrogen fixation on some ecotypes of alfalfa plants.

Regulation of cysteine residues in LsrB proteins from Sinorhizobium meliloti under free‐living and symbiotic oxidative stress

“…A chromatin immunoprecipitation (ChIP) assay using LsrB from S. meliloti cells was performed to determine whether LsrB binds directly to the gshA promoter in vivo. The P gshA fragment was precipitated from Rm1021 cells, which was similar to the P lrp3 positive control (Tang et al ., ), though little or no DNA fragments precipitated when using lsrB 3m mutant cells (Fig. H).…”

“…The sensitivity of lsrB mutants to GSSG prompted us to investigate the expression of lsrB and downstream genes in response to oxidative stress. Accordingly, the LsrB target gene lrp3 (Tang et al ., ) and ROS‐related genes, including katA and gshA (Lu et al ., ), were induced in Rm1021 by treatment with GSSG. Compared to Rm1021, the lsrB 3m and ΔlsrB mutants exhibited decreased induction of lrp3 , katA and gshA in response to GSSG (Fig.…”

“…OxyR has been found to regulate the expression of katA , but it is not required for symbiotic nitrogen fixation (Jamet et al ., ; Luo et al ., ). LsrB, another LysR family regulator from Sinorhizbium melilot i, has been reported to modulate the formation of alfalfa nitrogen‐fixing nodules, and regulates the expression of gshA , gshB1 and lpsCDE (Luo et al ., ; Lu et al ., ; Tang et al ., ). Therefore, it could be a new ROS regulator.…”

“…The nodules induced by a S. meliloti lsrB deletion mutant showed premature senescence, indicating impaired bacteroid differentiation (Tang et al ., ). LsrB directly regulates the expression of genes involved in lipopolysaccharide biosynthesis (Tang et al ., ); this regulator is associated with glutathione production and with induction of katA in an OxyR‐dependent manner (Lu et al ., ). Here, we report a direct relationship among alfalfa nodule development, ROS accumulation and LsrB regulation.…”

Identification of a gene encoding a sugar transporter as a potential target of a novel LysR‐type transcriptional regulator rluD from rhizobium required for symbiotic nodulation in soybean (Glycine max Merr.)

“…As the genomes from several rhizobia species have been sequenced, an increasing number of LysR‐type transcriptional regulators (LTTR), as well as nodD regulators, have been annotated (Del Cerro et al., ; Pérez‐Montaño et al., ; Tang, Wang, & Luo, ). More than 90 genes encoding LysR regulatory proteins have been predicted since the genome sequence of S. meliloti 1021 was published in 2001 (Galibert et al., ).…”