The dual roles of AlgG in C‐5‐epimerization and secretion of alginate polymers in Pseudomonas aeruginosa

Summary Pseudomonas aeruginosastrains causing chronic pulmonary infections in cystic fibrosis patients produce high levels of alginate, an exopolysaccharide that confers a mucoid phenotype. Alginate is a linear polymer ofd‐mannuronate (M) and variable amounts of its C‐5‐epimer,l‐guluronate (G). AlgG is a periplasmic C‐5‐epimerase that converts polyd‐mannuronate to the mixed M+G sequence of alginate. To understand better the role and mechanism of AlgG activity, a mutant was constructed in the mucoid strain FRD1 with a defined non‐polar deletion ofalgG. Instead of producing poly mannuronate, thealgGdeletion mutant secreted dialysable uronic acids, as does a mutant lacking the periplasmic protein AlgK. High levels of unsaturated ends and the nuclear magnetic resonance spectroscopy pattern revealed that the small, secreted uronic acids were the products of extensive polymer digestion by AlgL, a periplasmic alginate lyase co‐expressed with AlgG and AlgK. Thus, AlgG is bifunctional with (i) epimerase activity and (ii) a role in protecting alginate from degradation by AlgL during transport through the periplasm. AlgK appears to share the second role. AlgG and AlgK may be part of a periplasmic protein complex, or scaffold, that guides alginate polymers to the outer membrane secretin (AlgE). To characterize the epimerase activity of AlgG further, thealgG4allele of poly mannuronate‐producing FRD462 was shown to encode a protein lacking only the epimerase function. The sequence ofalgG4 has a Ser‐272 to Asn substitution in a serine–threonine‐rich and conserved region of AlgG, which revealed a critical residue for C‐5‐epimerase activity.