The unique conjugation system of IncHI3 plasmid MIP233.

The conjugation system of the IncHI3 plasmid MIP233 was studied using a transfer-derepressed Tn5-insertion mutant. The conjugative pili of this plasmid were short pointed rods resembling rigid pili, with a well-defined modal length. Unlike plasmids with rigid pili, the MIP233 mutant mediated a surface + liquid conjugation system. The pili were serologically different from all known pilus types including H pili, and did not act as receptors for any known pilus-specific bacteriophage. They converted the surface conjugation system of RP4 to a surface + liquid one. Antiserum to pili of the mutant plasmid inhibited transfer of the wild-type plasmid MIP233, demonstrating that it contained only one transfer system.