Isolation and partial characterization of regulatory mutants of the pyrimidine pathway in Salmonella typhimurium.

Mutants of Salmonella typhimurium affected in the regulation of pyrimidine biosynthesis were isolated by two methods. The first involved screening for bacteria able to feed a pyrimidine-requiring indicator strain, and the second involved selection for bacteria simultaneously resistant to two pyrimidine analogues, 5-fluorouracil and 5-fluorouridine, in a S. typhimurium strain unable to degrade 5-fluorouridine. Among the mutants isolated by these methods are constitutive mutants, producing high levels of pyrimidine biosynthetic enzymes in the presence or absence of pyrimidines, and feedback modified mutants, in which aspartate transcarbamylase is partially desensitized to its inhibitor, cytidine triphosphate. No fully desensitized mutant has been found. The partially desensitized character cotransduces with the pyrB locus, that of aspartate transcarbamylase. The constitutive character has been determined in a few cases to be localized in the region of leu and pro on the Salmonella map.