Studies on Some Enzymes of Alginic Acid Biosynthesis in Azotobacter vinelandii Grown in Continuous Culture

When a mutant of Azotobacter uinelandii was grown in continuous culture the amount of exocellular polysaccharide produced was dependent on both the dissolved oxygen tension (d.0.t.) and the carbon source: sucrose supported alginate synthesis in phosphate-limited medium whereas sorbitol did not. Changes in the specific activities of two of the key enzymes of alginate biosynthesis (phosphomannose isomerase and GDPmannose pyrophosphorylase), measured in extracts of cells grown with sucrose under a range of d.0.t. values, were reflected by the observed changes in alginate production; the activity of GDPmannose dehydrogenase was unchanged. A similar correlation between the specific activities of these enzymes and the rate of alginate production was observed during a transition from sorbitol to sucrose as the sole carbon source, but in this experiment the activity of GDPmannose dehydrogenase also increased with increasing alginate production. After prolonged continuous cultivation on sucrose the mutant gradually lost the ability to produce alginate. The key enzymes of alginate biosynthesis could not be detected in extracts of this non-alginate-producing strain, which had also lost the ability to encyst. These results support the suggestions that alginate formation is controlled by derepression of key biosynthetic enzymes and that alginate plays an important role in encystment