GENETIC INSTABILITY IN AUXOTROPHS OF SALMONELLA TYPHIMURIUM REQUIRING CYSTEINE OR METHIONINE AND RESISTANT TO INHIBITION BY 1,2,4-TRIAZOLE

Abstract

Triazole-resistant (Trz^r) derivatives of six cysteine- or methionine-requiring (Cym^-) mutants of Salmonella typhimurium were isolated. Some of the derivatives of each mutant (CTS) were prototrophic, i.e., Cym^- was suppressed. In every case suppression was initially unstable, Cym^- auxotrophs being segregated at high frequency, although Trz^r was stable. After several subcultures on selective medium, CTS strains were classified as either persistently unstable or stabilized. The unstable strains segregated Cym^- auxotrophs at frequencies of 50-70%, whereas the stabilized strains segregated at frequencies of less than 1%. All suppressed strains had a stable Trz^r marker co-transducible with cysA. However, there was a correlation between the class of CTS strain and Cym^- phenotype. The stabilized strains were Cym⁺, whereas the unstable strains were Cym^-. Acriflavin and ethidium bromide increased segregation in the unstable strains, suggesting the involvement of a plasmid. The stabilized strains were refractory to the curing agents. There was no detectable change in the quantity or quality of the S. typhimurium cryptic plasmid. The Trz^r phenotype of the CTS strains suggested that Trz^r mutations were of the stable TrzA type. It is suggested that correction of the Cym^- lesions in CTS strains results from an insertion within the cysCDHIJ gene cluster of a DNA species originating in the cysALKptsHI region of the S. typhimurium chromosome.