The enzyme GDP-mannose dehydrogenase (GMD) is encoded by the gene, and previous genetic studies have indicated that it is a key regulatory and committal step in the biosynthesis of the polysaccharide alginate. In the present study the gene has been cloned into the broad-host-range expression vector pMMB66EH and GMD overexpressed in mucoid and genetically-related non-mucoid strains of The metabolic approach of P. J. Tatnell, N. J. Russell & P. Gacesa (1993), 139, 119-127, has been used to investigate the subsequent effect of GMD overexpression on the intracellular concentrations of the key metabolites GDP-mannose and GDP-mannuronate, which have been related to GMD activity and total alginate production. The overexpression of in mucoid and non-mucoid strains resulted in elevated GMD activities compared to wild-type strains; there was a concomitant reduction in GDP-mannose concentrations and greatly increased GDP-mannuronate concentrations. However, significantly, alginate biosynthesis was detected only in mucoid strains and GMD overexpression resulted in only a marginal increase in exopolysaccharide production. The GDP-mannuronate concentrations in mucoid strains which overexpressed GMD were always significantly greater than those of GDP-mannose, indicating that GMD was no longer the major kinetic control point in the biosynthesis of alginate by these genetically-manipulated strains. The small but significant increase in alginate production by such strains together with the increased GDP-mannuronate concentrations is interpreted as meaning that a later enzyme of the alginate pathway has become the major kinetic control point and now determines the extent of alginate production. This study has provided direct metabolic evidence that GMD is the key regulatory enzyme in alginate biosynthesis in


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