Composite circular forms of R factor deoxyribonucleic acid molecules.

Two R factors, one (R15) conferring resistance to streptomycin and sulfonamide (SM(r)SU(r)) and the other (222/R3) to streptomycin, sulfonamide, and chloramphenicol (SM(r)SU(r)CM(r)), were transferred to a Proteus mirabilis strain, and deoxyribonucleic acid (DNA) extracted from these strains was subjected to density-gradient centrifugation. R15-DNA formed a single satellite band at a density of 1.709 g cm(-3). Electron microscopy of samples from this band showed circular molecules of one type, with a contour length of 18 mum (35 x 10(6) daltons). 222/R3-DNA formed a satellite band with three peaks at densities 1.708, 1.711 and 1.717 g cm(-3). Electron micrographs revealed circular structures from each band with contour lengths, respectively, of 29 (54 x 10(6) daltons), 36 (68 x 10(6) daltons), and 6 mum (12 x 10(6) daltons). "Supertwisted" forms of several molecular species were found. It is suggested that 222/R3 DNA comprises either a single 36-mum molecule or two individual molecules, 29 and 6 mum in length, and that this may reflect the evolutionary development of R factors.