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Physical Properties and Mechanism of Transfer of R Factors in Escherichia coli

¹ Department of Radiology, Yale University, New Haven, Connecticut 06510
Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut 06510
Department of Epidemiology and Public Health, Yale University, New Haven, Connecticut 06510

ABSTRACT

The physical properties of F-like and I-like R factors have been compared with those of the wild-type F factor in Escherichia coli K-12 unmated cells and after transfer to recipient cells by conjugation. The F-like R factor R538-1drd was found to have a molecular weight of 49 x 10⁶, whereas the molecular weight of the I-like R factor R64drd11 was 76 x 10⁶. The wild-type F factor, F1, had a molecular weight of 62 x 10⁶. When conjugation experiments are performed by using donor strains carrying these derepressed F-like or I-like R factors, the transferred deoxyribonucleic acid can be isolated as a covalently closed circle from the recipient cells. This circular deoxyribonucleic acid was characterized by making use of the observation that the complementary strands of these R factors can be separated in a CsCl-poly (U, G) equilibrium gradient. The results of the strand-separation experiments show that only one of the complementary strands of the R factor is transferred from the donor to the recipient. With both the F-like and I-like R factors, this strand is the heavier strand in CsCl-poly (U, G). These results indicate that even though F-like and I-like R factors differ greatly in many properties (phage specificity, size, compatability, etc.), they are transferred by a similar mechanism.

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