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J Bacteriol. 1974 February; 117(2): 619-630
Copyright © 1974 American Society for Microbiology. All Rights Reserved.

Molecular Nature of Two Nonconjugative Plasmids Carrying Drug Resistance Genes

^a Department of Microbiology, School of Medicine, University of Washington, Seattle, Washington 98195

ABSTRACT

Two nonconjugative R-plasmids, N-SuSm and N-Tc, have been characterized. Both were of relatively small size (5 x 10⁶ to 6 x 10⁶ daltons) and present in multiple copies within their respective bacterial hosts. N-SuSm possessed a guanine plus cytosine content of 55%, whereas N-Tc was 49% guanine plus cytosine. Although these plasmids were inherently nontransmissible they could be mobilized by a large variety of transfer agents including Ent, Hly, and K88. The fi^– transfer factors tested were far more likely (about 200x) to mobilize these nonconjugative plasmids than were the fi⁺ transfer factors tested. Although the mobilization phenomenon was not found to be associated with a detectable level of direct stable recombinational union between N-SuSm or N-Tc with a transfer factor, we were able to demonstrate a low level of recombination between these replicons and a transfer factor by P1-mediated transduction. The isolation of recombinants between transfer factors and nonconjugative plasmids presumably represents one means by which unitary molecular types of R-plasmids arise and by which existing R-plasmids may acquire new resistance determinants.

¹ Present address: Rijks Instituut De Volksgezondheid, Anthonie van Leeuwenhoeklaan 9, Postbus 1, Bilthoven, The Netherlands.

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