Recombination between plasmids of incompatibility groups P-1 and P-2

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Abstract
R plasmids of incompatibility group P-2 are readily transmissible between Pseudomonas strains, but not to Escherichia coli or other enterobacteria, but those of group P-1 have a broad host range. P. aeruginosa donor strains carrying both a P-1 plasmid (RP1, RP4 or R751) and a P-2 plasmid (pMG1, pMG2, pMG5 or RPL11) were mated with E. coli K-12, and selection was imposed for resistance markers on the P-2 plasmids. Transconjugants were obtained at a low frequency, in which P-2 markers were expressed and were serially transmissible in E. coli together with P-1 markers. These plasmids had P-1 incompatibility properties, conferred susceptibility to phages active on P-1-carrying strains, and behaved on sucrose gradient centrifugation as unimolecular species of higher MW than the P-1 parent. Recombinant plasmid formation was independent of a functional Rec gene in both donor and recipient and, with R751, had a preferred site leading to loss of trimethoprim resistance. Interaction between insertion sequences may be involved. Plasmids of group P-2 recombine with R factors of another group separate in compatibility properties, host range and pilus type. Formation of such recombinants provides 1 pathway by which the genetic diversity of plasmids may have evolved.