The conjugative transposon Tn 925: enhancement of conjugal transfer by tetracycline in Enterococcus faecalis and mobilization of chromosomal genes in Bacillus subtilis and E. faecalis

The effects of tetracycline on transfer of the conjugative, tetracycline-resistance transposon, Tn 925, as well as the ability of the transposon to promote the transfer of chromosomal genes was examined in Enterococcus faecalis and Bacillus subtilis. To test for chromosomal transfer, multiply-marked strains of each organism, each carrying a single chromosomal copy of Tn 925, were mated on filters with suitable recipient strains, under conditions where transformation and transduction were precluded. In both cases, transfer of a variety of chromosomal genes, at frequencies comparable to the frequency of Tn 925 transfer, was detected readily. The presence of Tn 925 in one of the members of the mating pair was absolutely required for chromosomal transfer, but transfer of Tn 925 did not accompany every chromosomal transfer event. The results were consistent with a mating event resembling a type of cell fusion, allowing for extensive recombination between the genomes of the mating partners. Growth of Tn 925-containing donor cells in the presence of tetracycline increased the transfer frequency of Tn 925 by about tenfold in E. faecalis, but not in B. subtilis.