Inversion of Transfer Modes and Sex Factor-Chromosome Interactions in Conjugation in Escherichia coli

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Abstract
A study was made of the mating properties of an unusual system of interconvertible donor strains of Escherichia coli K-12: Ra-1, Ra -2, and RaF+. The Ra-1 and Ra-2 strains are Hfr strains whose origins are widely separated on the chromosome and whose transfer modes proceed in the opposite direction from one another. When Ra-1 cells were mated with females, a small fraction of the donors transferred markers via the Ra-2 mode. This effect was enhanced by preconjugal ultraviolet (UV) treatment of the Ra-1 cells. Among the survivors of UV- treated Ra-1 cells, a few stable Ra-2 cells were found. When Ra-2 cells were used as the donors, some of them were found to mate via toe Ra-1 mode, in analogy with the Ra-1 -to-Ra-2 alteration with inversion of F mentioned above. Related experiments suggested that toe inversion occurs by detachment of the F factor from one Hfr origin locus, ollowed by reassociation of the F factor with the other Hfr origin locus. Both the Ra-1 and Ra-2 strains reverted spontaneously to an F+strain, calledRaF+. Cultures of RaF+ cells were found to mate primarily according to the Ra-1 and Ra-2 transfer modes, with smaller contributions also coming from transfer modes with origins else-where on the chromosome in a way which is similar to the transfer of markers from a normal F+ strain. The RaF+ sex factor was found to be wild type, whereas the chromosome was found to carry irregularities (sex factor affinity loci) atthe locations of the Ra-1 and Ra-2 origins. Only about 10% of the donor capacity of the RaF+ strain was due to stable spontaneous Hfr cells in culture of RaF+ cells.