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Thc ability of polynucleotide ligase to eliminatc single-strand breaks in yirradiated DNA was studied. Thc irradiatcd DNA was first Iabelled at the 5'-end of the brcak with "Pvphosphate und treated in-uitro with polynucleotide ligasc. In phosphoester-bonds joined by thc ligasc, thc incorporated 32p_ phospbate is resistant to subsequent phosphatase treatment. The number of sealed breaks, dctcctcd in t his way, was compared with the total number of single-strand breaks measured in the analytical ultracentrifuge. In DNA with 0•4-2•0 radiation-induced single-strand brcaks per 1000 nucleotides, up to 40 per cent of thc brcaks can be joined directly by the ligase.

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