An in vitro system has been developed to test whether colicin E2 possesses DNase activity. Purified colicin E2 preparations introduced one single-strand scission in supercoiled lambda phage DNA. Glycerol gradient fractionation of colicin E2 supports the association of in vitro action with in vivo cell-killing activity. Colicin E2 preparations also attacked superhelical SV40 DNA yielding open circles and fragments and single-stranded fd DNA molecules causing one or more endonucleolytic breaks. The possible role of contaminating nucleases in the activity of colicin E2 preparations is discussed.

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