Alternate forms of the resistance factor R1 in Proteus mirabilis.

The resistance factor R1 may exist in either of two stable physical states in Proteus mirabilis PM-1. In one case, the R1 deoxyribonucleic acid (DNA) has a buoyant density of 1.711 g/cm(3) and replicates under stringent control. Cells harboring R1 in this form may transfer drug resistance by conjugation. In the other case, R1 DNA shows two buoyant density classes at 1.707 and 1.714 g/cm(3). The 1.714 g/cm(3) component is replicated under a degree of relaxed control, and strains carrying this form generally cannot transfer drug resistance by conjugation. Intracellular amounts of the R factor-coded enzyme, chloramphenicol acetyltransferase, did not correspond to amounts of plasmid DNA in Proteus, and the enzyme was present in lower amounts than in Escherichia coli. It is proposed that the two states of R1 in Proteus may represent stable associated and dissociated forms of the plasmid.