Abstract
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.
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