The genetic and structural basis of two distinct terminal side branch residues in stewartan and amylovoran exopolysaccharides and their potential role in host adaptation

Abstract

SummaryStewartan and amylovoran exopolysaccharide (EPS) produced by the plant pathogenic bacteria Pantoea stewartii and Erwinia amylovora are virulence factors in the cause of Stewart's vascular wilt and fire blight. The biosynthesis of amylovoran and stewartan is encoded by a set of homologous operons that have been partially characterized, although some annotations are solely on the basis of sequence homology. The major distinguishing features of these two EPS forms are the presence of a terminal pyruvate in amylovoran and glucose in stewartan, even though the gene systems to account for both are conserved and present in each bacterium. This study explores the genetic, structural and functional differences of amylovoran and stewartan, and their potential role in host adaptation. We report that the pyruvyl transferase gene in P. stewartii is non-functional, while the terminal glucosyl transferase is catalytically active. Conversely, in E. amylovora, the homologous glucosyl transferase activity appears to be relatively ineffective, while the pyruvyl transferase function predominates. We also show that the terminally pyruvylated versus glucosylated EPS require specific repeating unit translocases (Wzx). We discuss the evolutionary, functional and biological implications of the terminally pyruvylated and glucosylated polymers and their potential contribution to plant and insect host adaptation.

Proteomic Analysis of the Quorum-Sensing Regulon in Pantoea stewartii and Identification of Direct Targets of EsaR

“…The mglB gene is part of a tricistronic operon coding for components of methyl-galactoside transporter and functions as a galactose chemoreceptor and the periplasmic binding component for the transporter in E. coli (38). Galactose in an important component of stewartan (39). The rpsB gene codes for a 30S ribosomal subunit protein S2p.…”

Wzx flippases exhibiting complex O‐unit preferences require a new model for Wzx–substrate interactions

“…Wzx 1 was severely deficient at flipping O units with α(1,3)‐linked Abe p , Tyv p or Par p residues, while Wzx 2 was moderately deficient when translocating O units with a β(1,3)‐linked Par f residue (Table ). Wang, Yang, and von Bodman () previously proposed that two distinct Wzx pairs in Pantoea stewartii and Erwinia amylovora were required to translocate a pair of exopolysaccharides differing only by a Glc or pyruvate terminal side‐branch moiety, but did not directly confirm the role of each Wzx nor test the degree of specificity for each structure (Table ). Hence, this study is the first to unambiguously demonstrate that Wzx substrate preference can be sensitive enough to be affected by minor alterations to the structure and/or linkage type of a single O‐unit residue.…”

Enzymatic Transformations Involved in the Biosynthesis of Microbial Exo‐polysaccharides Based on the Assembly of Repeat Units

“…[42,48] Further experiments suggested the use of different Wzx proteins for the selective transport of repeatu nits with an identicalb ackboneb ut modified side chains in several bacteria. [49,50] Just recently,t hese findings were updated by complementation experiments that showed the necessity of the initiating sugar'sb eing directly linked to the undecaprenyl pyrophosphate anchor.M odified repeat units were only very inefficiently translocated, if at all. [51] More research will be necessary to completely describe the mode of action and substrate specificity of Wzx proteins.…”