Composite Circular Forms of R Factor Deoxyribonucleic Acid Molecules

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Abstract
Two R factors, one (R15) conferring resistance to streptomycin and sulfonamide (SMrSUr) and the other (222/R3) to streptomycin, sulfonamide, and chloramphenicol (SMrSUrCMr), 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 × 106 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 × 106 daltons), 36 (68 × 106 daltons), and 6 μm (12 × 106 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.

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