The nature of genetic instability in auxotrophs of Salmonella typhimurium requiring cysteine or methionine and resistant to inhibition by 1,2,4-triazole.

We tested the hypothesis that unstable suppression of auxotrophy in triazole-resistant derivatives of Cym- mutants of Salmonella typhimurium is due to reversible insertion at the Cym- site of genetic material originating in the cysALKptsHI region. We have shown that the unstable phenotype was co-transducible with markers in the cysCDHIJ region. The suppression of the Cym phenotype was recA dependent and frequencies of segregation were affected by UV irradiation. Restored enzyme activity in suppressed strains was determined by wild-type enzyme, suggesting that the unstable regions are located in cys gene regulatory regions. These results support the hypothesis. In contradiction, we found no evidence for a deletion in the cysALKptsHI region.