Plasmids R68 and R68.45 were transferred from Pseudomonas aeruginosa to Escherichia coli by conjugation. R68.45 was able to mobilize the E. coli chromosome from different origins at a frequency of about lO−6/donor cell. With R68, no transfer of chromosomal genes could be detected. Plasmid R68.45 differs from its parent R68 only by an additional DNA segment, 2120 bp long, located close to the kanamycin resistance gene. By restriction enzyme analysis it was shown that the additional DNA segment of R68.45 is a duplication of a pre-existing DNA region of R68. The duplicated region is characterized by the following sequence of restriction sites: A–310 bp–SmaI–70 bp–PstI–795 bp–PstI–15 bp–KpnI–540 bp–HpaI–370 bp–B.
The endpoints A and B of the duplicated region were determined by a heteroduplex experiment between HindIII linearized molecules of R68 and R68.45. It is proposed that this duplication found in R68.45 is responsible for its chromosome mobilizing ability.