Sinorhizobium meliloti SyrA Mediates the Transcriptional Regulation of Genes Involved in Lipopolysaccharide Sulfation and Exopolysaccharide Biosynthesis

Abstract

Sinorhizobium meliloti is a gram-negative soil bacterium found either in free-living form or as a nitrogenfixing endosymbiont of leguminous plants such as Medicago sativa (alfalfa). S. meliloti synthesizes an unusual sulfate-modified form of lipopolysaccharide (LPS). A recent study reported the identification of a gene, lpsS, which encodes an LPS sulfotransferase activity in S. meliloti. Mutants bearing a disrupted version of lpsS exhibit an altered symbiosis, in that they elicit more nodules than wild type. However, under free-living conditions, the lpsS mutant displayed no change in LPS sulfation. These data suggest that the expression of lpsS is differentially regulated, such that it is transcriptionally repressed during free-living conditions but upregulated during symbiosis. Here, I show that the expression of lpsS is upregulated in strains that constitutively express the symbiotic regulator SyrA. SyrA is a small protein that lacks an apparent DNA binding domain and is predicted to be located in the cytoplasmic membrane yet is sufficient to upregulate lpsS transcription. Furthermore, SyrA can mediate the transcriptional upregulation of exo genes involved in the biosynthesis of the symbiotic exopolysaccharide succinoglycan. The SyrA-mediated transcriptional upregulation of lpsS and exo transcription is blocked in mutants harboring a mutation in chvI, which encodes the response regulator of a conserved two-component system. Thus, SyrA likely acts indirectly to promote transcriptional upregulation of lpsS and exo genes through a mechanism that requires the ExoS/ChvI twocomponent system.When nitrogen is limiting, leguminous plants enter into symbioses with members of the genera Rhizobium, Bradyrhizobium, Mesorhizobium, Azorhizobium, and Sinorhizobium (collectively called rhizobia) resulting in the formation of nodules. Within the nodule, differentiated cytoplasmic rhizobia called bacteroids reduce molecular dinitrogen to ammonia. Bacterial colonization of the nodule requires morphological alteration of epidermal cells called root hairs, resulting in the formation of a curled structure referred to as a shepherd's crook. Shepherd's crook formation is followed developmentally by the formation of an infection thread, a tubular ingrowth of the root hair that penetrates the plant. The infection thread is occupied by the bacteria, allowing their entry into the plant interior.

How rhizobial symbionts invade plants: the Sinorhizobium–Medicago model

“…The B. abortus bvrS/bvrR system may be required for virulence, because it controls the expression of several outer membrane proteins and is involved in modulating the properties of bacterial lipopolysaccharide 162,167 . It has not been possible to construct null mutants of the S. meliloti homologues of these genes, exoS and chvI, indicating that they are essential 32,168 . However, a mutant that produces a constitutively active ExoS sensor kinase is partially impaired in its ability to infect root hairs (see main text) [31][32][33] .…”

The Sinorhizobium meliloti SyrM Regulon: Effects on Global Gene Expression Are Mediated by syrA and nodD3

“…Changes common to all three transcriptomes include increased expression of the exo genes, sinI N-acyl-L-homoserine lactone synthetase, eglC glycanase, and genes of unknown function (see Data set S1 in the supplemental material). Ptrp-syrA was previously shown to increase expression of exoY-, exoH-and exoF-uidA gene fusions (69). In S. meliloti strains bearing an intact expR, SinI is required for quorum sensing via long-chain acyl homoserine lactones; sinI mutants do not show swarming motility and are less efficient at forming nodules than the wild type (70,71).…”

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

“…Therefore, LsrB is a new LysR family regulator that is associated with LPS production. Moreover, LsrB appears to modify only the amount of LPS, not its constituents, as is the case with SyrA and TtsI (15,16).…”

“…SyrA in S. meliloti and TtsI in S. fredii NGR234 contribute to the regulation of LPS O-antigen synthesis (15,16). It also has been reported that the transcription elongation factor GreA participates in LPS core biosynthesis in S. meliloti 1021 and S. fredii HH103 (7).…”

“…A small periplasmic regulator, SyrA, is coupled with the ExoS/ChvI two-component system to regulate expression of the LPS sulfotransferase gene, lpsS, in S. meliloti 1021 (12,15). In S. fredii NGR234, the type III secretion system regulator TtsI controls the biosynthesis of rhamnose and a rhamnose-rich component (rhamnan) of LPS (16).…”