A COMBINATION OF 3 MUTATIONS, DCD, PYRH, AND CDD, ESTABLISHES THYMIDINE (DEOXYURIDINE) AUXOTROPHY IN THYA(-TYPHIMURIUM() STRAINS OF SALMONELLA)

The dam gene of Salmonella typhimurium was originally identified as a gene involved in dUMP synthesis (C. F. Beck et al., J. Bacteriol. 129: 305-316, 1977). In the genetic background used in their selection, the feint acquisition of a dcd (dCTP deaminase) and a dam mutation establ ished a condition of thymidine (deoxy uridine) auxotrophy. In this stu dy, we show that dam is identical to pyrH, the gene encoding UMP kinas e. The level of UMP kinase activity in the dam mutant was found to be only 30% of that observed for the dum(+) strain. Thymidine prototrophy was restored to the original dam dcd mutant (KP1361) either by transd uction using a pyrH(+) donor or by complementation with either of two pyrH(+)-carrying plasmids. Thymidine auxotrophy could be reconstructed in the dum(+) derivative (KP1389) by the introduction of a mutant pyr H allele. To define the minimal mutational complement necessary to pro duce thymidine auxotrophy in thyA(+) strains, a dcd:Km null mutation w as constructed. In the wild-type background, dcd::Km alone or in combi nation with a pyrH (dum) mutation did not result in a thymidine requir ement. A third mutation, cdd (cytidine-deoxycytidine deaminase), was r equired together with the dcd and pyre mutations to impart thymidine a uxotrophy.