Transposon-mediated conjugational transmission of nonconjugative plasmids

When coresident with conjugative plasmid pNC21, the nonconjugative deletion F-prime pJC59, which retains the F transfer origin oriT, was transmitted to transconjugants at a frequency comparable to that of pNC21. In addition, pJC59 was transmitted as an independent plasmid, physically separate from pNC21, an example of plasmid donation. In contrast, two plasmids that are derived from F and deleted for the oriT site, pJC61 and pML31, were transmitted at frequencies 10(4) lower than that of pNC21. This low-frequency transmission was associated with the appearance of a new plasmid in the transconjugants. In the case of pML31, we determined that this new plasmid was a recombinant composed of pNC21 and pML31, the latter flanked by two copies of transposable element Tn3. We believe that this recombinant plasmid was formed as an intermediate in the transposition of Tn3 from pNC21 to pML31 and was the vehicle for conjugational transmission of pML31 genes by a process known as plasmid conduction.