CONSTRUCTION AND CHARACTERIZATION OF 2 LEXA MUTANTS OF SALMONELLA-TYPHIMURIUM WITH DIFFERENT UV SENSITIVITIES AND UV MUTABILITIES

Salmonella typhimurium has a SOS regulon which resembles that of Esche richia coli. recA mutants of S. typhimurium have already been isolated , but no mutations in lexA have been described Set. In this work, two different lexA mutants of S. typhimurium LT2 have been constructed on a sulA background to prevent cell death and further characterized, The lexA552 and lexA11 alleles contain an insertion of the kanamycin resi stance fragment into the carboxy- and amino-terminal regions of the le xA gene, respectively. SOS induction assays indicated that both lexA m utants exhibited a LexA(Def) phenotype, although SOS genes were appare ntly mere derepressed in the lexA11 mutant than in the lexA552 mutant. Like lexA(Def) of E. coli, both lexA mutations only moderately increa sed the UV survival of S. typhimurium, and the lexA552 strain was as m utable as the lexA(+) strain by UV in the presence of plasmids encodin g MucAB or E. coli UmuDC (UmuDC(Ec)). In contrast, a lexA11 strain car rying any of these plasmids was nonmutable by UV, This unexpected beha vior was abolished when the lexA11 mutation was complemented in trans by the lexA gene of S. typhimurium. Tile results of UV mutagenesis cor related well with those of survival to UV irradiation, indicating that MucAB and UmuDC(Ec), proteins participate in the error-prone repair o f UV damage in lexA552 but riot in lexA11, These intriguing difference s between the mutagenic responses of lexA552 and lexA11 mutants to UV irradiation are discussed, taking into account the different degrees t o which the SOS response is derepressed in these mutants.