Physiology and genetics of isocitritase and the malate synthases of Escherichia coli

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Two malate synthases are known to occur in Escherichia coli: one of them malate synthase A allows, with isocitritase, the growth on acetate, the other malate synthase G, the growth on glycolate as sole carbon sources. Mutants impaired in their growth on acetate (aceA, aceB) or on glycolate (glc) are characterized by their, growth rate on various carbon sources, by specific activities of isocitritase and malate synthase and by their relative content in each type of the two malate synthases. aceA and aceB mutants are deficient in isocitritase and malate synthase A respectively; mutant glc lacks the malate synthase G. The aceB mutation impairs growth on acetate only when present in a gle mutated strain. In strains of wild type for the genes concerned, growing on acetate, malate synthase A constitutes approximately 60% of the total activity of malate synthase, the rest being malate synthase G; on glycolate, the activity is nearly entirely represented by malate synthase G. Isocitritase and malate synthase A activities appear in similar relative amounts through different growth conditions, particularly, during growth on limited carbon source (chemostat); in such conditions, they allow the growth of a glucose negative strain (glu) even on glucose as sole carbon source. The location of aceA, aceB and glc mutations on the genetic map are established by sexual crosses and by phage 363 transductions. The frequencies of cotransduction between aceA or aceB and known loci (metB, glu-1, argH2, purD, metA, malB) completed by three point test analysis, lead to a detailed genetic map of this region (Fig. 3); aceA and aceB are located close together, between metA and malB. The glc mutation (deficiency in malate synthase G) happened in the strain HfrDV21 simultaneously with a translocation of the sexual factor of the parent strain Hfr 30SOMA4. The glc locus and the sexual factor of this new Hfr are located between the known loci argG and his, close to the marker serA with which glc is cotransducible. HfrDV21 transfers the genetic markers pheB, purC and his in the same order as the Hfr KL16 but approximately 8 minutes earlier. The results, together with the properties of the strain constitutive for isocitritase and malate synthase A, point to the occurrence of an ace operon (aceA, aceB) regulated by repression-derepression. Malate synthase G seems to be so easily inducible (by glyoxylate?) that it allows growth on acetate of a strain deficient in malate synthase A. The regulatory properties and physiological roles of the two isoenzymes suggest that the inducible malate synthase G would be more primitive than the repressible malate synthase A.