Molecular cloning of recombinant DNA : proceedings of the Miami winter symposia, January 1977 / edited by W.A. Scott, R. Werner ; sponsored by the Department of Biochemistry, University of Miami, School of Medicine, Miami, Florida.

Date:
1977
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Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Molecular cloning of recombinant DNA : proceedings of the Miami winter symposia, January 1977 / edited by W.A. Scott, R. Werner ; sponsored by the Department of Biochemistry, University of Miami, School of Medicine, Miami, Florida. Source: Wellcome Collection.

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    RECOMBINATION BETWEEN BACTERIAL PLASMIDS LEADING TO THE FORMATION OF PLASMID MULTIMERS John Bedbrook and Fred Ausubel Biological Laboratories, Harvard University We report here the formation of plasmid multi- mers in E. coli. Our experiments show that pMB9 DNA (and other plasmid DNAs) exists as various dis¬ crete sizes in ree hosts. Electron microscopic and restriction enzyme analysis indicates that the larger forms of pMB9 are covalently closed circular molecules composed of tandem repeats of the smallest size pMB9 DNA. To determine the role of the host recombination genes in plasmid multimer formation, we transformed ree or ree strains with plasmid molecules of a single size, isolated supercoiled DNA from the transformants, and fractionated the isolated DNA on agarose gels where plasmid multimers migrate as discrete bands. These experiments show that: 1) Multimer formation occurs at high frequency in rec+, recB~C~, and recF~ hosts. 2) Multimer form¬ ation is not detected in recA~ hosts and occurs at reduced frequency in recB~C~F~ hosts. 3) Multimers propagate themselves stably in recA~ hosts and multimer formation is reversible in rec+ hosts. 4) pMB9 forms hybrids with pML21, a plasmid which contains a limited region of homology with pMB9, in rec*^ but not in ree hosts. We conclude that plas¬ mid multimers arise by a single reciprocal recombin¬ ation event occuring at regions of homology between plasmid molecules. 241