The genetic control of anaplerotic reactions in Escherichia coli.

Three independent mutants induced with ethylmethane- sulphonate and selected for their inability to grow on acetate, were shown by enzymic assays in comparison with their wild-type parents and revertants to be devoid of isocitrate lyase. Genetic mapping by interrupted conjugation, recombination analysis and phage transduction showed each of these lesions to lie within the same gene locus situated at 78.3 min on the E. coli linkage map in the order met A, icl, pgi. Furthermore, the occurrence of Acetate recombinants from intragenic crosses between these mutants demonstrated their heteroallelism and located their sites within the icl locus in the order metA, icl-1, icl-2,icl-3. Mutants lacking a second anaplerotic enzyme, PEP-synthase were also obtained through E.M.S. mutagenesis but were selected for their inability to grow on lactate, pyruvate or alanine. Using the previously described mapping techniques the structural gene specifying this enzyme was shown to lie in close proximity to the aroD marker at 32.5 min on the E. coli linkage map. These latter mutants were then employed in the isolation of spontaneously occurring id regulatory mutants (iclR), which, by their constitutive production of isocitrate lyase, permitted the growth of these strains on lactate but neither on pyruvate nor acetate plus pyruvate. By interrupted mating and three-point transduction, the iclR lesions within two such independent mutants were mapped and shown to lie adjacent to the icl structural gene in the order metA, icl, iclR. Furthermore, "cis-trans" tests employing a merodiploid of this region demonstrated the transdominance of iclR~.' over iclR and showed this lesion to mark the site of an icl regulator locus. These results together with the location of the glyoxylate cycle malate synthase (masA) in the position masA, icl iclR (Vanderwinkel & de Vlieghere, I968) and the domonstration of the co-ordinate formation (Kornberg, I96I, I966) and de-repression (Vanderwinkel, Liard, Ramos & Wiame, I963) of these enzymes provide both structural and biochemical evidence for the occurrence of a glyoxylate cycle operon.