The effect of the relative concentrations of citrate and glucose on the regulation of key enzymes of the direct oxidative, phosphorylative, Entner–Doudoroff and pentose-cycle pathways of glucose metabolism in Pseudomonas aeruginosa has been investigated in continuous culture under conditions of NH4+-limitation. For comparison isocitrate dehydrogenase and aconitase were also assayed. Measurements were made for steady-state and transient conditions and the effect of growth rate was also studied. When cells grew on 75mm-citrate the glucose concentration had to attain 6–8mm before significant induction of enzymes of glucose metabolism occurred; the specific activities increased further as the result of both raising the glucose concentration to 30mm and then subsequently lowering the citrate to 60mm and then to 45mm. The specific activities of the glucose enzymes increased immediately during the transient period between the steady states characteristic of growth on 6mm- and 8mm-glucose, the increase continuing for about two doubling times. The converse experiment of adding increasing citrate concentrations to 45mm-glucose medium revealed an immediate induction of the citrate-transport system, oxidation of citrate following the increase in citrate concentration up to 8mm. Between 8mm- and 16mm-citrate a marked repression of gluconate, glucose 6-phosphate and 6-phosphogluconate dehydrogenases and the Entner–Doudoroff enzymes occurred. Increased growth rate in citrate medium resulted in decreased specific activities of glucose 6-phosphate dehydrogenase and isocitrate dehydrogenase. Increased growth rate in citrate–glucose medium gave decreased specific activities of isocitrate dehydrogenase and aconitase whereas the activities of some of the glucose enzymes decreased initially but then increased at the highest growth rate (0.5h-1), at which a marked increase in glucose utilization occurred. These observations accord with the regulation of glucose enzymes by induction with glucose or its metabolites and repression by citrate or its metabolic products.

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