F-Mediated Conjugation, F Strains, and Hfr Strains of Salmonella typhimurium and Salmonella abony

Some wild-type lines of Salmonella spp. contain plasmids which are self-transmissible by conjugation (25). However, most such plasmids have very low frequencies of self-transmission owing to repression of transfer functions; this situation has been extensively analyzed by Datta et al. (6). In addition, most plasmids give infrequent chromosome mobilization because of the low frequency of plasmid-chromosome recombination. A system for chromosome mobilization was established by Smith et al. (40) for Salmonella typhimurium (official designation, Salmonella enterica serovar Typhimurium) by using colicin factors. The system was based on their observation that conjugative colicin factors newly transmitted to a cell are transiently derepressed and give very high frequencies of subsequent transfer to recipient cells. This system of Col-factor-mediated conjugation (called HFC) used ColI and ColE1 factors and gave recombination frequencies of up to 1/10 cells tested; this allowed the construction of a partial linkage map of S. typhimurium (41). Stably derepressed colicin factors have subsequently been isolated (25), and some stable insertions of colicin factors into the chromosome of Escherichia coli have been obtained. However, the majority of genetic analysis by conjugation in both E. coli K-12 and S. typhimurium has been done using the F factor from E. coli K-12. This plasmid is well suited for genetic analysis because it is derepressed for F function and because homologous recombination between the plasmid and the chromosome gives rise to Hfr strains with the plasmid stably inserted in the chromosome. The F factor of E. coli K-12 was transferred from E. coli to S. typhimurium LT2 by Zinder (51) and to Salmonella abony by Mäkelä et al. (23). Hfr strains of S. typhimurium (30, 35, 37, 51) and S. abony (22) were isolated. These strains of Salmonella are not hybrids between E. coli and Salmonella spp. for chromosomal genes; they carry the F factor from E. coli, but all the data indicate that they carry no chromosomal genes from E. coli. Merodiploids are common among the progeny of S. typhimurium-E. coli intergeneric crosses. This merodiploidy is usually maintained through F′ factors, but in some cases, the extra DNA is inserted into the recipient chromosome (15, 16, 19, 29). Merodiploids of these types are not considered in this report.