A family of modular type mannuronan C-5-epimerase genes controls alginate structure in Azotobacter vinelandii

The L-guluronic acid residues in the Azotobacter vinelandii polysaccharide alginate originate from a post-polymerization reaction catalysed by the enzyme mannuronan C-5-epimerase (ME). We have previously reported the cloning and expression of an A. vinelandii gene encoding this enzyme, and we show here that the organism encodes at least four other ME genes originating from a common ancestor gene by a complex rearrangement process. The biological function of the corresponding enzymes is probably to catalyse the formation of alginates with a variety of physical properties. This model may explain the origin of the structural variability found in alginates isolated both from prokaryotic and eukaryotic organisms. The A. vinelandii enzymes may also potentially be useful for certain medical and biotechnological applications of this commercially important polysaccharide.