SUMMARY: DNA breakdown was detected 3 to 4 min. after addition of colicin E2 to sensitive cells; inhibition of cell division followed 5 to 10 min. later, but inhibition of DNA synthesis was observed only after several more minutes. Adsorption of E2, which takes place even at 4°, led to the formation of a specific surface complex (I). Complex I did not promote DNA breakdown. We suggest that the transition from this complex to a surface complex (II) which promoted DNA breakdown depended upon several factors which include temperature, concentration of E2, specific membrane proteins and, under certain conditions, high concentrations of extracellular KHPO. The formation of complex II did not depend on concomitant DNA or protein synthesis. The continued promotion of DNA breakdown by complex II and its associated nuclease was blocked by inhibition of energy metabolism. In addition, the removal of E2 from the cell surface by trypsin treatment during the early stages of the process greatly decreased the rate of DNA breakdown. E2-induced DNA breakdown, which appears to commence from a limited number of chromosomal sites, proceeded normally in UVr, RecB, RecC, Hsr, Hss, PolA and in several tsDNA replication mutants.


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