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We have isolated three strains of Klebsiella aerogenes that failed to show repression of glutamine synthetase even when grown under the most repressing conditions for the wild-type strain. These mutant strains were selected as glutamine-independent derivatives of a strain that is merodiploid for the glnA region and contains a mutated glnF allele. The mutation responsible for the Gln+ phenotype in each strain was tightly linked to glnA, the structural gene for glutamine synthetase, and was dominant to the wild-type allele. These mutations are probably lesions in the control region of the glnA gene, since each mutation was cis-dominant for constitutive expression of the enzyme in hybrid merodiploid strains. Strains harboring this class of mutations were unable to produce a high level of glutamine synthetase unless they also contained an intact glnF gene, and unless cells were grown in derepressing medium. This study supports the idea that the glnA gene is regulated both positively and negatively, and that the deoxyribonucleic acid sites critical for positive control and negative control are functionally distinct.

Isolation of Klebsiella aerogenes mutants cis-dominant for glutamine synthetase expression