Biosynthesis of the repeating units of the exopolysaccharides amylovoran from Erwinia amylovora and stewartan from Pantoea stewartii
The causative agent of fire blight, Erwinia amylovora, synthesizes the exopolysaccharide (EPS) amylovoran, an important pathogenicity factor. Similarly, Pantoea stewartii, needs the related exopolysaccharide, stewartan, to cause Stewart's bacterial wilt of sweet maize. These acidic complex EPSs mask the recognition of the pathogens E. amylovora and P. stewartii by plant defense reactions and promote bacterial growth and movement in planta. The E. amylovora ams gene cluster determines the biosynthesis of the amylovoran EPS repeating unit, which is comprised of four galactose and one glucuronic acid residues. Stewartan is synthesized by the P. stewartii cps gene products. Functions of Ams proteins were analyzed in an in-vitro system with EDTA-treated cells. Cross-complementation of an amsD mutant with the cpsFG genes from P. stewartii resulted in chimeric EPS with glucose instead of galactose as the second sugar residue in the backbone. AmsG adds the first galactose and AmsD the next one. The ability of CpsF to attach the next galactose residue to the backbone of the chimeric EPS suggests a similar role for AmsE. Evidence for the possible roles of other glycosyltransferases in the assembly of the repeating units of amylovoran and stewartan is presented. (C) 2011 Elsevier Ltd. All rights reserved.