Temperature-Sensitive Mutants for the Replication of Plasmids in Escherichia coli : Requirement for Deoxyribonucleic Acid Polymerase I in the Replication of the Plasmid ColE 1

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Abstract
An Escherichia coli mutant (polA1), defective in deoxyribonucleic acid (DNA) polymerase I, (EC 2.7.7.7) is unable to maintain colicinogenic factor E1 (ColE1), whereas several sex factor plasmids are maintained normally in this strain. polA1 mutant strains containing these sex factor plasmids do not exhibit a readily detectable plasmid-induced polymerase activity. A series of E. coli mutants that are temperature sensitive for ColE1 maintenance, but able to maintain other plasmids, were isolated and shown to fall into two phenotypic groups. Mutants in one group are defective specifically in ColE1 maintenance at 43 C, but exhibit normal DNA polymerase I activity. Mutations in the second group map in the polA gene of E. coli, and bacteria carrying these mutations are sensitive to methylmethanesulfonate (MMS). Revertants that were selected either for MMS resistance or the ability to maintain ColE1 were normal for both properties. The DNA polymerase I enzyme of two of these mutants shows a pronounced temperature sensitivity when compared to the wild-type enzyme. An examination of the role of DNA polymerase I in ColE1 maintenance indicates that it is essential for normal replication of the plasmid. In addition, the presence of a functional DNA polymerase I in both the donor and recipient cell is required for the ColV-promoted conjugal transfer of ColE1 and establishment of the plasmid in the recipient cell.