Rhizobium leguminosarum bv. trifolii PssP Protein Is Required for Exopolysaccharide Biosynthesis and Polymerization

Abstract

Rhizobium leguminosarum bv. trifolii produces an acidic exopolysaccharide (EPS) that is important for the induction of nitrogen-fixing nodules on clover. Recently, three genes, pssN, pssO, and pssP, possibly involved in EPS biosynthesis and polymerization were identified. The predicted protein product of the pssP gene shows a significant sequence similarity to other proteins belonging to the PCP2a family that are involved in the synthesis of high-molecular-weight EPS. An R. leguminosarum bv. trifolii TA1 mutant with the entire coding region of pssP deleted did not produce the EPS. A pssP mutant with the 5' end of the gene disrupted produced exclusively low-molecular-weight EPS. A mutant that synthesized a functional N-terminal periplasmic domain but lacked the C-terminal part of PssP produced significantly reduced amounts of EPS with a slightly changed low to high molecular form ratio. Mutants affected in the PssP protein carrying a stable plasmid with a constitutively expressed gusA gene induced nodules on red clover that were not fully occupied by bacteria. A mutant with the entire pssP gene deleted infected only a few plant cells in the nodule. The pssP promoter-gusA reporter fusion was active in bacteroids during nodule development.

Lipoprotein PssN of Rhizobium leguminosarum bv. trifolii: Subcellular Localization and Possible Involvement in Exopolysaccharide Export

“…An analogous result was previously obtained for strain TA1. Deletion of the pssP gene encoding an MPA protein resulted in an Exo Ϫ mutant (43). Based on this, it was concluded that mutation in pssN might be lethal for TA1.…”

“…PssP protein (MPA1 family) comprises 2 transmembrane (TM) segments and a large periplasmic loop, which shows high propensity for folding into coiled-coil structures possibly engaged in interactions with the outer membrane component of the EPS transporter. PssP is required for exopolysaccharide biosynthesis and determination of the degree of EPS polymerization (43). PssL is a Wzx-like protein that most probably acts in flipping the lipid-linked oligosaccharides to the outer leaflet of the cytoplasmic membrane (44).…”

The Determinants of the Actinorhizal Symbiosis

“…Disruption of genes involved in EPS synthesis usually resulted in more severe impairments of symbiosis than in the cases of other surface polysaccharides. Although numerous works on EPS mutants are reported (22,25,27,51,52,57,63,83,96,117,120), their phenotypes can be summarized into abortion of the infection threads followed by formation of non-nitrogenfixing nodules with no or less bacteroids. The defective nodules exhibited features characteristic of plant defense responses such as thickened cell wall and accumulation of callose, phenolic compounds, phytoalexin and chitinase (71,80) suggesting their role in suppression of defense responses.…”

Exopolysaccharide Biosynthesis in Rhizobium leguminosarum: From Genes to Functions

“…These observations are also true for R. leguminosarum model. As discussed above, mutations in the genes controlling the polymerization and transport of EPS in the Rlt TA1 strain could contribute to the production of LMW EPS [93].…”

“…1) and assigned to be involved in polymerization and export of EPS [42,92,93]. The PssT protein has been predicted to be a Wzy-like protein that together with PssL might be responsible for Wzx/Wzy-like-dependent EPS polymerization and translocation.…”