Studies on some enzymes of alginic acid biosynthesis in mucoid and nonmucoidAzotobacter chroococcum strains

Measurements of enzymes involved in alginate biosynthesis were straightforward in mucoid (alginate-positive)Azotobacter chroococcum ATCC 4412 crude extracts. At the stationary growth phase, where the production of the exopolysaccharide was greatest, the enzymes phosphomannose isomerase and GDP-mannose pyrophosphorylase increased markedly, whereas phosphomannomutase and GDP-mannose dehydrogenase kept the high activity levels measured in the acceleration growth phase. In nonmucoid (alginatenegative)A. chroococcum andA. vinelandii strains, the activities of phosphomannose isomerase and GDP-mannose pyrophosphorylase were rather low or, in some cases, undetectables. Except inA. chroococcum MCD1, which exhibited a low activity, phosphomanomutase was high in the nonmucoidAzotobacter strains, and GDP-mannose dehydrogenase reached a significant activity level in two out of four nonmucoid strains tested. The results suggest that derepression of phophomannose isomerase and GDP-mannose pyrophosphorylase is asine qua non condition for alginate formation byA. chroococcum.