5-Fluoroorotate-resistant mutants of Salmonella typhimurium.

Spontaneously occurring mutants of Salmonella typhimurium resistant to 5-fluoroorotate (5-FOA) were isolated. One class of mutant showed marked derepression of pyrimidine biosynthetic enzymes and had the unusual property of being unable to grow on nutrient agar. However, when the osmotic strength of nutrient agar was increased, the mutants were able to grow. The genetic basis for the osmotic fragility and elevated pyr enzyme synthesis was the result of mutations affecting pyrH, encoding the enzyme uridine 5'-monophosphate kinase.