Alternate Forms of the Resistance Factor R1 in Proteus mirabilis

Abstract

The resistance factor Rl may exist in either of two stable physical states in Proteus mirabilis PM-1. In one case, the Rl deoxyribonucleic acid (DNA) has a buoyant density of 1.711 g/cm' and replicates under stringent control. Cells harboring Rl in this form may transfer drug resistance by conjugation. In the other case, Rl DNA shows two buoyant density classes at 1.707 and 1.714 g/cm'. The 1.714 g/cms 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 Rl in Proteus may represent stable associated and dissociated forms of the plasmid.Although the F-like R factors exist predominantly as a single molecular species in Escherichia coli (3,14), their organization may be more complex in Proteus mirabilis (4, 13, 17). This greater complexity of structure, moreover, is generally accompanied by the relaxed replication of the R factor (4, 16), so that the number of copies of the plasmid increases to a degree determined in part by the physiological state of the culture. Different explanations have been offered to describe these phenomena at the molecular level. Nisioka et al. (13) separated 222/R3 plasmid deoxyribonucleic acid (DNA) from bulk Proteus chromosomal DNA by isopycnic centrifugation in CsCl. The R factor DNA formed three peaks, with buoyant densities of 1.717, 1.711, and 1.708 g/cm5. Examination of this mnaterial by electron microscopy indicated that the peaks were composed of circular molecules with mean contour lengths of 6, 36, and 29 ,m, respectively. These lengths suggested that the 36-ism element was a composite of the other two, a proposal that is consistent with the buoyant density of this molecule. Cohen and Miller (4) reported similar results in Proteus for R factors Rl and R6. In addition, these investigators demonstrated the intercellular transfer of the 29-Am element by conjugation without concomitant transfer of drug resistances (4, 5). This molecular species is then the transfer 'Present address: factor (RTF), and the 6-Am piece (the r plasmid) presumably includes the drug resistance genes.Both Cohen and Miller (3) and Nisioka et al. (14) have concluded that in E. coli nearly all the R factor DNA is in the form of a single molecular species. Judging from its buoyant density and contour length, this species appears to be identical to the composite structure found in Proteus.A different proposal for the molecular organization of R factors in Proteus has been offered by Rownd and Mickel (17). In their view, R DNA consists of RTF and r determinants, which may associate to form pqpulations of linear molecules of varying sizes. Each size class would represent molecules formed by the covalent linkage of an RTF and an integral number of r determinants. Cohen a...