Cloning of an exclusion-determining fragment of the IncI plasmid, R144.

By cloning a distinct 8 MDa fragment of the IncI plasmid, R144, in the vector pACYC184, two recombinant plasmids were isolated. In these plasmids, pRAH303 and pRAH308, the inserted fragment was in opposite orientations. Both plasmids when present in a recipient strain caused a conjugation-specific exclusion in crosses with donor cells carrying the IncI plasmid R144. Some derivatives of the recombinant plasmids in which parts were deleted, or in which Tn5 transposons were inserted, appeared to be exclusion negative. Analysis in minicells of the gene products of such plasmids together with those of the original recombinant plasmids revealed that the presence of two proteins, with apparent molecular weights of 13,000 and 19,000 Da could be correlated with the exclusion phenomenon.