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Role of an Alginate Lyase for Alginate Transport in Mucoid Pseudomonas aeruginosa

Sumita Jain^1,2, and Dennis E. Ohman^1,3*

Department of Microbiology and Immunology, Medical College of Virginia Campus of Virginia Commonwealth University, Richmond, Virginia 23298-0678,¹ University of Tennessee Health Sciences Center, Memphis, Tennessee 38163,² McGuire Veterans Affairs Medical Center, Richmond, Virginia 23249³

Received 24 February 2005/ Returned for modification 1 May 2005/ Accepted 8 July 2005

The opportunistic pathogen Pseudomonas aeruginosa secretes a capsule-like polysaccharide called alginate that is important for evasion of host defenses, especially during chronic pulmonary disease of patients with cystic fibrosis (CF). Most proteins for alginate biosynthesis are encoded by the 12-gene algD operon. Interestingly, this operon also encodes AlgL, a lyase that degrades alginate. Mutants lacking AlgG, AlgK, or AlgX, also encoded by the operon, synthesize alginate polymers that are digested by the coregulated protein AlgL. We examined the phenotype of an algL mutation in the highly mucoid CF isolate FRD1. Generating a true algL mutant was possible only when the algD operon was under the control of a LacI^q-repressed trc promoter. Upon induction of alginate production with isopropyl-ß-D-thiogalactopyranoside, the algL mutant cells were lysed within a few hours. Electron micrographs of the algL mutant showed that alginate polymers accumulated in the periplasm, which ultimately burst the bacterial cell wall. The requirement of AlgL in an alginate-overproducing strain led to a new model for alginate secretion in which a multiprotein secretion complex (or scaffold, that includes AlgG, AlgK, AlgX, and AlgL) guides new polymers through the periplasm for secretion across the outer membrane. In this model, AlgL is bifunctional with a structural role in the scaffold and a role in degrading free alginate polymers in the periplasm.

Editor: V. J. DiRita

Present address: Department of Microbiology and Molecular Genetics, Harvard Medical School, Boston, MA 02446.

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