Isolation and characterization of pyrimidine mutants of Salmonella typhimurium altered in expression of pyrC, pyrD, and pyrE

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Abstract
Parental strains of Salmonella typhimurium having a specific pyr gene (pyrC, pyrD, pyrE) fused to the structural genes of the lac operon through the specialized transducing phage Mu dl (ApR lac) were used to construct themostable derivatives for purposes of conducting a genetic and biochemical characterization of the individual pyr genes. The direction of transcription of each pyr gene in relation to the current linkage map was defined with both pyrC and pyrE being transcribed counterclockwise and pyrD exhibiting clockwise transcription. Mutants displaying increased pyr gene expression were isolated employing a genetic strategy which is of general applicability. Among the mutants, only one isolate was found to possess a mutation which was unlinked to the specific pyr gene under study; the other isolates harbored linked mutations which were inferred to be cis-acting. Additional studies demonstrated that in conditions of severe pyrimidine limitation, further derepression could still occur in the mutant strains.

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