Genetic Structure of an R Factor Conferring Ampicillin Resistance

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An R factor carrying resistant genes, ampicillin (amp), tetracycline (tet), chloramphenicol (cml), dihydrostreptomycin (str), and sulfanilamide (sul), was analyzed by three genetic techniques: spontaneous segregation, transduetion, and recombination. The R factor was stable in Shigella and Escherichia coli, but in Salmonella the (cml. str. sul) genes were spontaneously lost at a high frequency. Almost all of the transductants with P1 contained the five drug markers, but a very small number of them carried both tet and amp genes, or the (amp. cml. str. sul) genes. In conjugal transfer from a strain carrying a single type of R factor to R+ recipient, the recombination took place between the amp and tet, amp and cml, and between the cml and str genes, at a frequency of 23.5, 39.5, and 3.0%, respectively. Crossover between the amp and transfer (tra) loci was not observed. A circular genetic model is proposed in which the genes are arranged in the order-tet9-tet1-tra. amp-cml-str-.

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