The physical characteristics of a mutant, RM201-2, capable of conferring high and stable ampicillion resistance was analyzed. The RM201-2 and its parent R-factor deoxyribonucleic acid (DNA) could be isolated as an extrachromosomal and covalently closed circular form. Their buoyant densities were both 1.712 g/cm3, and their molecular weights were about 82 × 106 and 64 × 106, respectively, when measured by CsCl and sucrose density gradient analyses. The contour lengths by electron microscopy were 35.9 ± 0.6 and 31.0 ± 0.6 μm, respectively. By using the extracted R-factor DNA, the mutant and parent characters were transformable to another Escherichia coli strain. The mutant R factor showed an increased amount of DNA even after conjugal transfer to Proteus. An increase in the size of R-factor DNA was thus considered to be the cause of the high level of ampicillin resistance
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