The Rhizobium meliloti exoK gene and prsD/prsE/exsH genes are components of independent degradative pathways which contribute to production of low‐molecular‐weight succinoglycan

Abstract

SummaryWhen grown on medium supplemented with the succinoglycan-binding dye, Calcofluor, and visualized under UV light, colonies of Rhizobium meliloti (Sinorhizobium meliloti ) exoK mutants produce a fluorescent halo with a delayed onset relative to wild-type colonies. By conducting transposon mutagenesis of exoK mutants of R. meliloti and screening for colonies with even more severe delays in production of these fluorescent halos, we identified three genes, designated prsD, prsE, and exsH, which are required for the eventual production of fluorescent halos by exoK colonies. Nucleotide sequence indicates that the prsD and prsE genes encode homologues of ABC transporters and membrane fusion proteins of Type I secretion systems, respectively, whereas exsH encodes a homologue of endo-1,3-1,4-␤-glycanases with glycine-rich nonameric repeats typical of proteins secreted by Type I secretion systems. The exoK gene and the prsD /prsE /exsH genes were shown to be components of independent pathways for production of extracellular succinoglycan degrading activities and for production of low-molecular-weight succinoglycan by R. meliloti. Based on these results, we propose that ExsH is a succinoglycan depolymerase secreted by a Type I secretion system composed of PrsD and PrsE, and that the ExsH and ExoK glycanases contribute to production of low-molecular-weight succinoglycan.

The Symbiosis Regulator CbrA Modulates a Complex Regulatory Network Affecting the Flagellar Apparatus and Cell Envelope Proteins

“…We identified several genes encoding known or putative secreted extracellular proteins as differentially expressed in the cbrA::Tn5 mutant (Table 2), including the well-characterized succinoglycan modification gene exoK (89) and nex18 (55). The ExoK endoglycanase promotes bacterial invasion of host tissues during infection thread growth by converting high-molecular-weight forms of succinoglycan to the symbiotically active low-molecular-weight forms (58,89).…”

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

“…The present study is the first to suggest a role for quorum sensing in the production of LMW succinoglycan. The endoglycanases ExsH and EglC, both of which play a role in the synthesis of the LMW fraction of succinoglycan required for the successful invasion of the plant host (49,66), seem to be controlled by quorum sensing. We have confirmed that indeed, the exsH gene is expressed at a higher level in the presence of the ExpR regulator and an active Sin system (see Table 5).…”

“…Two endoglycanase genes, exsH (SMb20932) and eglC (SMa1587) (see Table 3, group A or F, experiments 1 and 4), are expressed at a higher level in the presence of the sin-AHLs. Both of these genes were previously shown to be major contributors to the production of LMW succinoglycan, but their regulation remained unclear (66). The expression of exsH was analyzed by real-time PCR with S. meliloti strains that retain or lack one or two components of the Sin/ExpR system.…”

Structures of Exopolysaccharides Involved in Receptor-mediated Perception of Mesorhizobium loti by Lotus japonicus

“…This resulted in increased production of LMM EPS, thus suggesting that O-acetylation may play a regulatory role in EPS length modifications. In contrast, an exoH mutant defective in succinylation produced only HMM EPS (37,52,53). The exoZ mutant was able to establish symbiosis with alfalfa but with reduced efficiency of infection thread formation and reduced nodule invasion (36,54), whereas the exoH mutant was unable to initiate infection thread formation.…”