Summary: Mutants of deficient in growth on nitrogen sources other than NH were compared with mutants of a similar phenotype isolated from . In addition to N and some amino acids (glutamate, alanine, proline, arginine), mutants of and strain AD2 were unable to utilize NO as sole nitrogen source for growth. Under conditions of nitrogen starvation, mutants of both species lacked the methylammonium (ammonium) uptake system, which was found in the wild-type strains under these conditions. The wild-type () genes complementing these mutants were isolated from gene banks of the respective species and localized to a 2·9 kb HI-I fragment in and to a 1·7 kb I fragment in . These two fragments hybridized strongly with each other, showing that they contain homologous sequences. Furthermore, the gene from fully restored the wild-type phenotype to Adg mutants of and vice versa. Inactivation of the chromosomal gene by insertion of an antibiotic-resistance cassette resulted in a severe inhibition of growth in rich medium and in minimal medium containing NH . This suggests that the gene is required for normal growth under all growth conditions.


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