SUMMARY: Pyrimidine biosynthesis in strain A126 was investigated. In this study, pyrimidine biosynthetic pathway mutant strains were isolated using both conventional mutagenesis and transposon mutagenesis. The resulting mutant strains were deficient for either aspartate transcarbamoylase, dihydroorotase or orotate phosphoribosyltransferase activity. Uracil, uridine or cytosine could support the growth of every mutant strain selected. In addition, the aspartate transcarbamoylase mutant strains could utilize orotic acid to sustain their growth while the orotidine-5′-monophosphate decarboxylase mutant strains grew slowly upon uridine 5′-monophosphate. The wild-type strain and the mutant strains were used to study possible regulation of pyrimidine biosynthesis in Dihydroorotase specific activity more than doubled after the wild-type cells were grown in orotic acid relative to unsupplemented minimal-medium-grown cells. Starving the mutant strains of pyrimidines also influenced the levels of several pyrimidine biosynthetic pathway enzyme activities.


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