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Conjugal Deoxyribonucleic Acid Replication by Escherichia coli K-12: Effect of Nalidixic Acid

^a University of Tennessee-Oak Ridge Graduate School of Biomedical Sciences and Biology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830

ABSTRACT

During the conjugal transfer of the R64-11 plasmid at 42 C from donor cells thermosensitive for vegetative deoxyribonucleic acid (DNA) synthesis to recipient minicells, the plasmids are conjugally replicated in the donor cells. This conjugal replication is inhibited by nalidixic acid, and the degree of inhibition is comparable to the reduction in the amount of plasmid DNA transferred to the recipient minicells in the presence of the drug. In addition, the size of DNA transferred to the minicells and the fraction of conjugally replicated DNA in the donor cells that can be isolated as closed-circular plasmid DNA under alkaline conditions are both reduced by nalidixic acid. When the drug is added to a mating that is underway, the rate of conjugal replication is immediately reduced. This change is accompanied by a reduction in the amount of conjugally replicated DNA in the donor cells that can be isolated as closed-circular plasmid DNA. Furthermore, conjugally replicated plasmid DNA that is not associated with the donor cell membrane becomes membrane bound after the addition of nalidixic acid.

¹ Present address: Section of Medical Genetics, Department of Medicine, Baylor College of Medicine, Houston, Tex. 77025.

² Present address: Department of Microbiology, University of Alabama in Birmingham, Birmingham, Ala. 35294.