Mechanisms of 5′-Inosinic Acid Accumulation by Permeability Mutants ofBrevibacterium ammoniagenes. III. Intracellular 55′-IMP Pool and Excretion Mechanisms of 55′-IMP

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Abstract
A series of mutants of Brevibacterium ammoniagenes selected for increased production of 5′-inosinic acid (5′-IMP) were analyzed for the size of their intracellular 5′-IMP and hypoxanthine (Hx) pools. All the mutants tested contained relatively large amounts of intracellular 5′-IMP in the earlier phase of growth irrespective of their 5′-IMP productivities, and the sizes of the pools corresponded fairly well to their 5′-IMP productivities. Furthermore, excretion of intracellular 5′-IMP by these mutants was measured by incubating the washed cells in a phosphate buffer at 30°C. In the absence of glucose in the reaction buffer, intracellular 5′-IMP of all the mutants except KY13102 was excreted partly as 5′-IMP and partly as Hx, after degradation, to the external buffer. However, accumulation of Hx in the external buffer was significantly repressed by glucose. Consequently in the case of the highest producer, KYI3369, all of the intracellular 5′-IMP was excreted as 5′-IMP without degradation in the presence of glucose. These results suggest the existence of two excretion systems of internal 5′-IMP de novo synthesized; a direct excretion system stimulated by glucose and an indirect excretion system coupled with degrading activity, which was repressed by glucose.