ColE1 plasmid mobility: essential and conditional functions.

Sequences essential for the conjugal transfer of ColE1 can be divided into a cis-acting site and a region encoding trans-acting products. Each of these was successively cloned into a non-transmissible plasmid vector. The resulting chimera was transmissible by the conjugative plasmids F'lac,pro (incFI) and R64drd11 (incIalpha). The sequences encoding colicin E1, immunity, and incompatibility were absent from this chimera: therefore they are not essential for the conjugal transmission of the ColE1 plasmid. In contrast to ColE1, however, the same chimera was deficient in conjugal transfer initiated by R751 (incP) and R388 (incW). This suggests that ColE1 sequences other than those cloned in the chimeric plasmid are necessary for its mobilization by R751 and R388. Three such regions were revealed by screening a series of ColE1 insertion mutants for transfer by R751 and R388. Two of these regions encode no other known function while the third is encoded by a region which overlaps the gene for colicin E1 itself.