Repair of DNA double-strand breaks in Escherichia coli, which requires recA function and the presence of a duplicate genome

Abstract

A method was devised for extracting, from cells of Escherichia coli K12, DNA molecules which sedimented on neutral sucrose gradients as would be expected for free DNA molecules approaching the genome in size. Gamma ray irradiation of oxygenated cells produced 0.20 DNA double-strand breaks per kilorad per 10⁹ daltons. Incubation after irradiation of cells grown in K medium, with four to five genomes per cell, showed repair of the double-strand breaks. No repair of double-strand breaks was found in cells grown in aspartate medium, with only 1.3 genomes per cell, although DNA single-strand breaks were still efficiently repaired. Cells which were recA⁻ or recA⁻recB⁻ also did not repair double-strand breaks. These results suggest that repair of DNA double-strand breaks may occur by a recombinational event involving another DNA double helix with the same base sequence.