Endonucleolytic incision of X irradiated deoxyribonucleic acid by extracts of Escherichia coli

Abstract

An endonuclease activity that reacts with X irradiated DNA is present in extracts of E. coli. By using centrifugal methods to monitor the conversion of the supercoiled, circular double stranded DNA for phage phiX 174 (replicative form) or PM2 to the relaxed circular form it was possible to quantitate the rate of radiation induced endonuclease sensitive sites in the DNA. For every single strand break induced by X rays under aerobic irradiation conditions, there is approximately one induced site sensitive to this endonuclease activity. Under irradiation conditions (addition of potassium iodide) that dramatically reduce rates of single strand breaks and 'alkali labile' lesions, the number of endonuclease sensitive sites relative to single strand breaks increases approximately 4 fold. This nuclease is present in several strains of E. coli B and K12, including mutants deficient in DNA polymerase I, recombination gene products (rec mutants), ultraviolet light incision enzyme (uvrA mutant), and endonuclease II. It is suggested that this endonuclease may be involved in an excision repair process for damages incurred in DNA by ionizing radiation.