contains two genes ( and ) encoding aconitase activities. An mutant was engineered by replacing the chromosomal gene by an internally deleted derivative containing a cassette. An double mutant was then made by transducing a previously constructed mutation into the strain. Western blotting confirmed that the AcnA and AcnB proteins were no longer produced by the corresponding mutants and PCR analysis showed that the chromosomal gene had been replaced by the disrupted gene. Aerobic and anaerobic growth in glucose minimal medium were impaired but not abolished by the mutation, indicating that the lesion is partially complemented by the gene, and growth was enhanced by glutamate. The double mutant would not grow on unsupplemented glucose minimal medium and although it responded to glutamate like a typical auxotroph under anaerobic conditions, under aerobic conditions no response to glutamate was observed before it was over-grown by ‘revertants’ lacking citrate synthase (). The double mutant retained a low but significant aconitase activity (<5% of wild-type), designated AcnC. Enzymological and regulatory studies with fusions indicated that AcnB is the major aconitase, which is synthesized earlier in the growth cycle than AcnA, and subject to catabolite and anaerobic repression.


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