Killing of Escherichia coli K-12 by near-ultraviolet radiation in the presence of hydrogen peroxide: role of double-strand DNA breaks in absence of recombinational repair.

Near-ultraviolet (NUV) radiation killing of Escherichia coli K-12 can be enhanced by a sub-lethal concentration of hydrogen peroxide. This can be divided into a "RecA-dependent" and "RecA-independent" synergistic killing action. Stationary phase wild-type and 8 closely related repair-deficient mutants were examined for their NUV sensitivities in the presence and absence of H2O2. All exhibited the "RecA-independent" synergism; i.e., H2O2 enhanced NUV lethality when RecA repair was not operating. The "RecA-independent" synergism did not result from destruction of repair enzymes. Very few DNA--protein crosslinks could be detected following NUV plus H2O2 treatment. However, double-strand (DS) DNA breaks were produced, apparently by conversion of closely spaced single-strand (SS) breaks on opposite strands. The correlation between DS-break formation and lethality in wild-type and a polA mutant indicates that the RecA-independent synergistic killing results from the conversion of SS into lethal DS breaks.