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Journal of Bacteriology, June 2003, p. 3515-3523, Vol. 185, No. 12
0021-9193/03/$08.00+0     DOI: 10.1128/JB.185.12.3515-3523.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

The Pseudomonas fluorescens AlgG Protein, but Not Its Mannuronan C-5-Epimerase Activity, Is Needed for Alginate Polymer Formation

Martin Gimmestad,1 Håvard Sletta,2 Helga Ertesvåg,1 Karianne Bakkevig,1 Sumita Jain,3 Sang-jin Suh,3 Gudmund Skjåk-Bræk,1 Trond E. Ellingsen,2 Dennis E. Ohman,3,4 and Svein Valla1*

Department of Biotechnology, Norwegian University of Science and Technology,1 SINTEF Applied Chemistry, Trondheim, Norway,2 Department of Microbiology and Immunology, Medical College of Virginia Campus of Virginia Commonwealth University,3 McGuire Veterans Affairs Medical Center, Richmond, Virginia4

Received 2 December 2002/ Accepted 27 March 2003

Bacterial alginates are produced as 1-4-linked ß-D-mannuronan, followed by epimerization of some of the mannuronic acid residues to {alpha}-L-guluronic acid. Here we report the isolation of four different epimerization-defective point mutants of the periplasmic Pseudomonas fluorescens mannuronan C-5-epimerase AlgG. All mutations affected amino acids conserved among AlgG-epimerases and were clustered in a part of the enzyme also sharing some sequence similarity to a group of secreted epimerases previously reported in Azotobacter vinelandii. An algG-deletion mutant was constructed and found to produce predominantly a dimer containing a 4-deoxy-L-erythro-hex-4-enepyranosyluronate residue at the nonreducing end and a mannuronic acid residue at the reducing end. The production of this dimer is the result of the activity of an alginate lyase, AlgL, whose in vivo activity is much more limited in the presence of AlgG. A strain expressing both an epimerase-defective (point mutation) and a wild-type epimerase was constructed and shown to produce two types of alginate molecules: one class being pure mannuronan and the other having the wild-type content of guluronic acid residues. This formation of two distinct classes of polymers in a genetically pure cell line can be explained by assuming that AlgG is part of a periplasmic protein complex.

* Corresponding author. Mailing address: Department of Biotechnology, NTNU Norwegian University of Science and Technology, N-7491 Trondheim, Norway. Phone: (47)73593320. Fax: (47)73591283. E-mail: svein.valla{at}biotech.ntnu.no.

Journal of Bacteriology, June 2003, p. 3515-3523, Vol. 185, No. 12
0021-9193/03/$08.00+0     DOI: 10.1128/JB.185.12.3515-3523.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.



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