SUMMARY: A nitrate non-utilizing mutant of () was isolated following UV mutagenesis and filtration enrichment. In preliminary reports, this mutant was designated , but it has now been renamed This mutant has a complex pleiotropic phenotype which affects most aspects of nitrogen utilization. The wild-type parent could use nitrate, nitrite, ammonium, urea, polyamines, and most amino acids and purines as sole source of nitrogen, but the mutant could use only putrescine, anthranilic acid, and the amino acids asparagine, aspartic acid, glutamic acid, glutamine, ornithine, proline and tyrosine. Most of these compounds could also serve as the sole source of both carbon and nitrogen. Addition of 1 mM-glutamine or 1 mM-proline to the medium resulted in a synergistic growth response if spermidine, arginine, citrulline, histidine, phenylalanine or tryptophan was present as the primary nitrogen source. Addition of 1 mM-ammonium to the medium repressed growth when putrescine, aspartic acid, asparagine, glutamic acid or tyrosine was the primary nitrogen source. The addition of ammonium did not repress growth when anthranilic acid, glutamine, ornithine or proline was the primary nitrogen source. The mutant grew more rapidly than its wild-type parent in race tubes on some complex media. The complexity and diversity of the alterations in nitrogen catabolism caused by the mutation suggest that this locus is important in the regulation of nitrogen catabolism by


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