SUMMARY: A proportion of wild-type strains of unable to recombine with K12 F— mutants can ‘infect’ them with the sex-differentiating character known as F. Of these strains, not all infect the same K12 F– mutants and the ease of transfer of F varies among them, suggesting that there are different F agents. This is supported by the finding that there are great variations in the transfer of unselected markers in crosses in which the only known difference is the origin of the F factor possessed by the F + parent. Colicinogenicity and F are frequently associated and all of nine strains producing colicin I possess F, but they are not identical, since the two properties have been separated by treatment with cobalt chloride.


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