The isolation and characterization of a mutant altered in l-amino oxidase regulation is reported. The previously isolated strain, which only synthesizes the glutamine synthetase α monomer and lacks the β monomer, was used as parental strain. A mutant derivative of strain was selected for its ability to grow on minimal medium in the presence of -methionine--sulfoximine (MSO), an inhibitor of glutamine synthetase activity. This mutant overcame the inhibitory effect of MSO by increasing the activity of -amino acid oxidase, an enzyme capable of degrading this compound. In contrast with the wild-type strain, the -amino acid oxidase of the MSO mutant was resistant to glutamine repression; in fact, it was induced by this amino acid but repressed by ammonium. This mutant is different from other nitrogen regulatory mutants reported and is only altered in the regulation of -amino acid oxidase. The MSO mutation is epistatic to -2 since the double mutant regulated the -amino acid oxidase in the same way as the single mutant.


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