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 μm (35 × 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 × 10 6 daltons), 36 (68 × 10 6 daltons), and 6 μm (12 × 10 6 daltons). “Supertwisted” forms of several molecular species were found. It is suggested that 222/R3 DNA comprises either a single 36-μm molecule or two individual molecules, 29 and 6 μm in length, and that this may reflect the evolutionary development of R factors. Images