Drug Resistance of Enteric Bacteria VIII. Chromosomal Location of Nontransferable R Factor inEscherichia coli

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Abstract
The R21(TC) factor, obtained by transduction of the R10(TC.CM.SM.SA) factor with phage ε to group ESalmonella, is not transferable by the normal conjugal process. However, when R21(TC)+transductants are infected with the F13factor, the nontransferable R21(TC) factor acquires transmissibility by conjugation. R21(TC)+conjugants ofEscherichia coliK-12, to which only the R21(TC) factor was transmitted by cell-to-cell contact from an F′ R+donor, were still unable to transfer their R21(TC) factor by conjugation. In crosses between Hfr and F−E. coliK-12 strains containing R21(TC), the gene responsible for tetracycline resistance was located on theE. coliK-12 chromosome betweenlacandpro, nearlac.