A selection procedure for uracil catabolism mutant strains involving indicator dye plates was developed. Using this method, a strain defective in uracil catabolism has been isolated in that was temperature-sensitive at 42°C where it required low concentrations of -carbamoyl-β-alanine, β-alanine or pantothenic acid for growth. An extract of the mutant strain degraded uracil at 37°C at a significantly diminished rate compared to that observed for the wild-type strain under the same growth conditions. The conversion of dihydrouracil to -carbamoyl-β-alanine was blocked at all temperatures examined in the mutant strain. By means of genetic analysis, the mutant strain was determined to be defective at two genetic loci. Transduction studies with bacteriophage P22 indicated that the gene is mutated in this strain, accounting for its β-alanine requirement. Episomal transfers between and the mutant strain provided evidence that the defect in uracil catabolism was located in another region of the chromosome.


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