BACTERIAL ALGINATES - BIOSYNTHESIS AND APPLICATIONS

Alginate is a copolymer of beta-D-mannuronic acid and alpha-L-guluroni c acid (GulA), linked together by 1-4 linkages. The polymer is a well- established industrial product obtained commercially by harvesting bro wn seaweeds. Some bacteria, mostly derived from the genus Pseudomonas and belonging to the RNA superfamily I, are also capable of producing copious amounts of this polymer as an exopolysaccharide. The molecular genetics, regulation and biochemistry of alginate biosynthesis have b een particularly well characterized in the opportunistic human pathoge n Pseudomonas aeruginosa, although the biochemistry of the polymerizat ion process is still poorly understood. In the last 3 years major aspe cts of the molecular genetics of alginate biosynthesis in Azotobacter vinelandii have also been reported. In both organisms the immediate pr ecursor of polymerization is GDP-mannuronic acid, and the sugar residu es in this compound are polymerized into mannuronan. This uniform poly mer is then further modified by acetylation at positions O-2 and/or O- 3 and by epimerization of some of the residues, leading to a variable content of acetyl groups and GulA residues. In contrast, seaweed algin ates are not acetylated. The nature of the epimerization steps are mor e complex in A. vinelandii than in P. aeruginosa, while other aspects of the biochemistry and genetics of alginate biosynthesis appear to be similar. The GulA residue content and distribution strongly affect th e physicochemical properties of alginates, and the epimerization proce ss is therefore of great interest from an applied point of view. This article presents a survey of our current knowledge of the molecular ge netics and biochemistry of bacterial alginate biosynthesis, as well as of the biotechnological potential of such polymers.