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The infection of different Hfr strains of Escherichia coli bearing derepressed R factors of the fi(+) or fi(-) type can result in the loss of the R factor and the conversion of the infected cells to the R(-) state. This extends earlier observations on the elimination of F' factors by bacteriophage M13 infection. Variability in the efficiency of this conversion can arise because of genetic factors independent of the R factor being eliminated. A fraction of the infected but unconverted R(+) cells were M13 carrier strains. The carrier state had an intracellular basis, and single R(+) cells could maintain the carrier state.
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