Consequences of interaction between F plasmid and a drug-resistance plasmid belonging to incompatibility group F1.

Two plasmids, pLK1 and pLK2, were derived from pIP218, an in vivo recombinant of plasmid F and the drug-resistance plasmid pIP176 (Cmr, Smr, Sur, Tcr). Of these two plasmids, pLK1 is 70 Mdaltons and carries the Tc-resistance determinant in a 7-Mdalton transposition element; pLK2 is 125 Mdaltons and carries Cm-, Sm- and Su-resistance determinants. The plasmid pLK1 resulted as a Tc-resistance segregaant of PIP218 during its coexistence with another plasmid, Co1E1-araC101, and pLK2 (125 Mdaltons) as a CmrSmrSur segregant during the conjugal transfer of pIP218. Both plasmids belong to the F1-incompatibility group, surface-exclude each other and Flac, and are derepressed for transfer. Incompatibility studies also indicated the preferential maintenance of pLK2 in hosts carrying either pLK2 and pLK1, or pLK2 and F'lac. An explanation of this phenomenon is provided. Furthermore, our data suggest the illegitimate recombination of the chromosomal lac genes with pLK1. In course of the incompatibility studies, the tet determinant was transposed from pLK1 into the chromosome, from the chromosome into the lac genes of an Flac plasmid, and from the Flac plasmid into another site on a second Flac plasmid.