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

Authors

  • Sumita Jain,

    1. Department of Microbiology and Immunology, Medical College of Virginia Campus of Virginia Commonwealth University, PO Box 980678, 1101 E. Marshall St., Rm. 5-051, Richmond, VA 23298-0678, USA.
    2. University of Tennessee Health Science Center, Memphis, TN, USA.
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    • Present address: Department of Microbiology and Molecular Genetics, Harvard Medical School, 200 Longwood Ave., Boston, MA 02115, USA.

  • Michael J. Franklin,

    1. Department of Microbiology and Center for Biofilm Engineering, Montana State University, Bozeman, MT, USA.
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  • Helga Ertesvåg,

    1. Department of Biotechnology, Norwegian University of Science and Technology, N-7491 Trondheim, Norway.
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  • Svein Valla,

    1. Department of Biotechnology, Norwegian University of Science and Technology, N-7491 Trondheim, Norway.
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  • Dennis E. Ohman

    Corresponding author
    1. Department of Microbiology and Immunology, Medical College of Virginia Campus of Virginia Commonwealth University, PO Box 980678, 1101 E. Marshall St., Rm. 5-051, Richmond, VA 23298-0678, USA.
    2. McGuire Veterans Affairs Medical Center, Richmond, VA, USA.
    • For correspondence at the first address. E-mail deohman@hsc.vcu.edu ; Tel. (+1) 804 828 9728; Fax (+1) 804 828 9946.

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Summary

Pseudomonas aeruginosa strains 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 of d -mannuronate (M) and variable amounts of its C-5-epimer, l -guluronate (G). AlgG is a periplasmic C-5-epimerase that converts poly d -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 of algG . Instead of producing poly mannuronate, the algG deletion 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, the algG4 allele of poly mannuronate-producing FRD462 was shown to encode a protein lacking only the epimerase function. The sequence of algG4 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.

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