Control of diauxic growth of Azotobacter vinelandii on acetate and glucose

Batch cultures of Azotobacter vinelandii were inoculated with cells pregrown on either acetate or glucose. When they were subsequently grown on a mixture of acetate and glucose, typical diauxic growth was observed, with preferential uptake of acetate in the first and glucose in the second phase of growth. Extracts from acetate-pregrown cells exhibited high acetate kinase activity in the first phase of growth. This activity decreased and activities of the two glucose enzymes glucose 6-phosphate dehydrogenase and glyceraldehyde 3-phosphate dehydrogenase increased in the second phase. Extracts from glucose-pregrown cells exhibited high initial activities of the two glucose enzymes, which decreased while acetate kinase activity increased in the first phase of growth. Again, in the second phase, activities of the two glucose enzymes increased and acetate kinase activity decreased. In any case, isocitrate dehydrogenase activity varied only slightly and unspecifically. The differences in enzyme activity and the constancy of isocitrate dehydrogenase were confirmed by experiments with either acetate- or glucose-limited chemostats. In chemostats in which both of the substrates were limiting, all of the enzymes displayed significant activities. Glucose 6-phosphate dehydrogenase activity was inhibited by acetyl coenzyme A and acetyl phosphate but not by acetate. It is proposed that diauxic growth is based on the control of enzymes involved in acetate or glucose dissimilation by which acetate or its metabolites control the expression and activity of glucose enzymes.