Regulation of glucose catabolism in Thiobacillus A2 grown in the chemostat under dual limitation by succinate and glucose

In contrast to its diauxic behaviour in batch culture, Thiobacillus A2 grew in chemostat culture using glucose and succinate as dual limiting substrates. Biomass production under dual limitations was the sum of that on single substrates with each substrate being oxidized and assimilated to similar extents in single and dual substrate-limited cultures. In glucose and glucose + succinate-limited cultures glucose was oxidized largely by the Entner-Doudoroff and pentose phosphate pathways, but other mechanisms also contributed and the ratios of pathways depended on substrate ratios and the previous substrate-history of the culture. Variations in specific activities of enzymes of carbohydrate metabolism following switches from single to mixed substrates were considerable, ranging from fourfold for fructose diphosphate aldolase to more than 200-fold for hexokinase, fructose diphosphatase, glucose 6-phosphate and 6-phosphogluconate dehydrogenases. Changes in specific activities occurred only over prolonged time periods in the chemostat, probably reflecting low concentrations of free substrates in carbon-limited cultures and consequent low levels of catabolite repression.