In vitro repair of single-strand breaks in γ-irradiated DNA by polynucleotide ligase

SummaryThe ability of polynucleotide ligase to eliminate single-strand breaks in γ-irradiated DNA was studied. The irradiated DNA was first labelled at the 5′-end of the break with 32P-phosphate and treated in vitro with polynucleotide ligase. In phosphoester-bonds joined by the ligase, the incorporated 32P-phosphate is resistant to subsequent phosphatase treatment. The number of sealed breaks, detected in this 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 breaks per 1000 nucleotides, up to 40 per cent of the breaks can be joined directly by the ligase.