Highly mobile DNA segment of IncI alpha plasmid R64: a clustered inversion region

When R64 DNA was digested with EcoRI, two DNA fragments not equimolar to the plasmid DNA were produced. A DNA region including these fragments was cloned (pKK009), and the pKK009 DNA sample was found to be a mixture of six or more DNA species with EcoRI, PstI, and AvaI cleavage sites at different positions, suggesting a complex rearrangement of DNA. When a part of the pKK009 DNA was removed by HindIII digestion, 33 different types of plasmids (pKK010-series plasmids) were obtained out of 58 clones tested, but no DNA rearrangement could be observed. On the basis of a comparison of the detailed restriction maps of these pKK010-series plasmids, we propose a model in which four DNA segments invert independently or in groups within the 1.95-kilobase region of R64, so that the arrangements of these four segments change randomly. The fixed pKK010-series plasmid DNA was again rearranged in the presence of R64, indicating that trans-acting gene function may be present to mediate the DNA rearrangement. The gene (tentatively designated as rci) was located on a 4.5-kilobase E9' fragment of R64.