title: Molecular nature of two nonconjugative plasmids carrying drug resistance genes.
abstract: Two nonconjugative R-plasmids, N-SuSm and N-Tc, have been characterized. Both were of relatively small size (5 x 10(6) to 6 x 10(6) 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.
mesh_terms: D002250:Carbon Radioisotopes; D002499:Centrifugation, Density Gradient; D003227:Conjugation, Genetic; D003596:Cytosine / Q000032:analysis; D004269:DNA, Bacterial / Q000032:analysis; D004352:Drug Resistance, Microbial; D004926:Escherichia coli / Q000032:analysis / Q000187:drug effects*; D005796:Genes; D006147:Guanine / Q000032:analysis; D008854:Microscopy, Electron; D008970:Molecular Weight; D009268:Nalidixic Acid / Q000494:pharmacology; D011995:Recombination, Genetic; D012475:Salmonella / Q000187:drug effects; D013307:Streptomycin / Q000494:pharmacology; D013449:Sulfonamides / Q000494:pharmacology; D013752:Tetracycline / Q000494:pharmacology; D013941:Thymine; D014161:Transduction, Genetic; D014316:Tritium
chemical_list: D002250:Carbon Radioisotopes; D004269:DNA, Bacterial; D013449:Sulfonamides; D014316:Tritium; D009268:Nalidixic Acid; D006147:Guanine; D003596:Cytosine; D013752:Tetracycline; D013941:Thymine; D013307:Streptomycin