Cold-sensitive Mutants of Salmonella typhimurium Defective in Uridine Monophosphate Kinase (pyrH)

Conditional mutants of Salmonella typhimurium carrying mutations in the structural gene encoding UMP kinase (pyrH) have been isolated. At 20° the mutants are unable to grow even in enriched medium, while they grow, although with somewhat reduced rates, in minimal medium at 37°. CMP kinase activity is not affected by the mutations indicating that the phosphorylation of CMP and UMP is carried out by different enzymes. When growing exponentially at 37° the mutants contain subnormal UTP pools and, as a consequence, derepressed levels of CMP kinase and aspartate carbamyltransferase. That the entire de novo biosynthetic pathway for UMP is derepressed is further indicated by the ability of the pyrH mutants to feed pyrimidine auxotrophs. Interrupted matings place pyrH at about 10 min on the S. typhimurium map. It cotransduces with the gene, pan, at a frequency of about 30% using the Escherichia coli phage Plkc. No cotransduction between pyrH and pan was found with the smaller Salmonella phage P22. The suggestion is made that the sole function of CMP kinase is to recycle products of turnover of mRNA.