ColE1 plasmid mobility and relaxation complex

Abstract

LIKE other small plasmids, ColE1 cannot promote its own conjugal transfer. Conjugative plasmids such as F′lac allow its efficient mobilisation: ColE1 is nonconjugative but mobile (Mob⁺) or transmissible. There is now strong evidence that ColE1 transfers actively and autonomously, rather than by integration into the conjugative plasmid or by passive diffusion in the mating aggregate. Passive diffusion is unlikely as many derivatives of ColE1 are transfer-deficient (Mob⁻) (refs 1–4), as are some other small plasmids. Also the finding that some cells acquire ColE1 but no conjugative plasmid seems to discount cointegrate transfer; so does the observation that some transfer-deficient F′lac mutants can still mobilise ColE1 (ref. 5). Here we show that ColE1 conjugal mobility requires both a specific site on the plasmid DNA and sequences specifying diffusible gene products. We propose that the site is at the transfer origin, which is also where ColE1 mobility protein(s) nick ColE1 in relaxation complex⁶. Translocation of this site to another normally non-mobile plasmid, allows its mobilisation when ColE1 mobility proteins are supplied in trans.