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SUMMARY: A series of Escherichia coli k 12 strains of various mating types was made resistant to colicin I (colI-r) and colicinogenic for this character (colI +). In the absence of the F sex factor, the number of ‘competent donors’ of colI in broth cultures of colicinogenic strains was about 1 %. Transfer of colI to non-colicinogenic strains reached about 7–10 % within 2 hr of mixing, and between 50 and 100 % after incubation overnight. Cells newly infected with colI transfer more efficiently, and can infect over 90 % of a recipient strain with colI within 1 hr of mixing (high-frequency colicino-geny transfer, HFC). When the F factor was present in the colicinogenic donor strain of a mixture, colI transfer was reduced; when F was present in the non-colicinogenic recipient, colI transfer was increased. The colicin I factor appeared to be transferred by a series of Hfr strains as an extrachromosomal element.
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