Physical maps of the two independently isolated Escherichia coli plasmids, pMB1 and ColE1, were prepared with 13 restriction endonucleases and compared. A 5.1 kilobase continuous region covering 55% of pMB1 and 75% of ColE1 was found to have similar, but non-identical, restriction maps. The differences in the maps of this region probably arose by localized mutational events rather than by major sequence rearrangements. The F-factor was found to mobilize pMB1 efficiently for conjugal transfer. A region on pMB1 required for its F-mediated transfer was mapped. Results of our study combined with results of other investigators suggest that pMB1 and ColE1 share functional properties such as colicin production, colicin immunity, mode of replication, and mobilization by the F-factor, and that the sequences required to code these functions are contained within the 5.1 kilobase homologous region.
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