Conjugational behaviour of N plasmids in Escherichia coli K12

Summary

R factors of the N incompatibility group apparently transfer at low frequencies (often 10^-6–10^-5 per donor cell) in 30 min liquid matings. However, the level of transfer is greatly increased when donor-recipient mixtures are held on solid medium selective for the recipient only, prior to transconjugant selection. Increase in transconjugant recovery is still donor-dependent at plating and therefore arises from mating on the plate. The process limiting N-mediated conjugation in liquid is probably mating pair formation, suggesting that the increase in yield of transconjugants may result from provision of a solid substrate for mating pair formation rather than from the delay in selection per se. The kinetics of N plasmid transfer on solid medium resemble those of transfer of a derepressed, rather than a repressed, F-like R factor. Preliminary experiments suggest that RP4, the prototype P incompatibility group plasmid, shows a stimulation of transfer on solid medium similar to the N plasmids studied.

We suggest that N, and probably P, plasmids are (like F and the col V's) naturally derepressed for fertility, but this is masked by the incompetence in liquid matings of donors carrying them.