Characterization of the ColE1 mobilization region and its protein products

A third of the 6.6 kb genome of ColE1 is devoted to mobilization (mob) genes necessary to promote its specific transfer in the presence of conjugative plasmids. The mob region is genetically complex: two mob genes are entirely overlapped by a third. Oligonucleotide-directed mutagenesis was used to insert an amber codon into one of the overlapped genes and make possible a full complementation analysis of mob. Four mob genes essential for mobilization by R64drd11 were thus identified. Fragments of mob were subcloned under control of the Ptac promoter in a suitable vector, overexpressed in minicells and the mobilization proteins visualized. A comprehensive alignment of the mob region of ColE1 with those of its close relatives ColK and ColA demonstrating that the four essential mob genes are conserved is also presented.