Mutant Hfr strains defective in transfer: restoration by F-like and I-like de-repressed R factors

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Abstract
SUMMARY: Mutants of Hfr strains ofEscherichia coliK12 defective in conjugation owing to failure to produce sex pili were isolated by resistance to F-specific phage. Tests of the ability of six de-repressed F-like R factors and four de-repressed I-like R factors to restore Hfr donor behaviour to these mutants indicated that there were several ways in which such restoration could occur. Of a total of 26 defective mutants, 12 were restored by F-like R factors. In these twelve, the function of the integrated F factor was evidently restored, because the sex pili contained F pilin subunits, distinguishable from the R pilin by serological tests. In contrast, among the four I-like R factors, only two were effective, butin all 26 defective mutants; the restored Hfr bacteria produced only I-like pili. The I-like sex factors, in restoring Hfr donor behaviour, did not therefore act by complementing the defective F.

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