SUMMARY: When cultures of strain 15 were irradiated with ultraviolet radiation, the bacteria were induced to form a bactericidal material, colicin-15. Colicin-15 was released by the lysis of bacteria which underwent a residual growth without division. The only strains of bacteria sensitive to the action of colicin-15 were derivatives of strain 15. Three colicin-resistant strains were obtained, all of which became simultaneously sensitive to every T-phage, while the colicin-sensitive parental strains were lysed only by T2. By stepwise reversal of the phage sensitivity pattern, it was possible to revert colicin-resistant mutants to colicin sensitivity again. Only certain mutations conferring resistance in concert to phages T1, T3, T4, T5 and T7 seemed to result in colicin sensitivity. A model which will account for these phenomena is presented.


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