Derepression of F factor function in Salmonella typhimurium.

In Salmonella typhimurium LT2 the F factor of Escherichia coli K-12 replicates normally but is repressed; Flac+ cells give no visible lysis on solid media with male-specific phages, low frequency transfer of Flac+ (0.001-0.007 per donor cell), few f2 infective centers (0.002-0.006 per cell), and they propagate male-specific phages to low titers. Thus they display a Fin+ (fertility inhibition) phenotype. This repression, owing to pSLT, a 60 Mdal plasmid normally resident in S. typhimurium, was circumvented by the following materials: (i) Flac+ plasmids from E. coli with mutations in finP or traO; (ii) a S. typhimurium line which had been cured of pSLT; (iii) pKZl, a KmR plasmid in the same Inc group as pSLT, which caused expulsion of pSLT and made Fin- lines; (iv) F-Fin- mutants which originated spontaneously and which are present in most Hfr strains of S. typhimurium. Strains which are derepressed for F function by the above methods give visible lysis on solid media with male-specific phages, ca. 1.0 Lac+ recombinants per donor cell in conjugal transfer, ca. 0.82 f2 infective centers per cell, over 80% of cells with visible F pili, and propagation of male-specific phages to high titer. These data confirm earlier observations that pSLT represses F by the FinOP system. In addition, it shows that there is no other mechanism which represses F function in S. typhimurium. If donor function is derepressed by one of the above methods, and if rough recipient strains are used, F-mediated conjugation in S. typhimurium LT2 is as efficient as in E. coli K-12.