Photodynamic effects of dyes on bacteria. V. Mutagenesis by acridine orange and 460-nm or 500-nm light in strains of Escherichia coli that differ in repair capability.

With acridine orange (AO) and monochromatic 460-nm light, the mutation rate of the wild-type strain of Escherichia coli (WP2) was 3-fold higher than that of the endonuclease-deficient strain WP2 (uvrA). In addition, the mutation rates of the recombination-deficient strains WP10 (recA) and Bs-1 (uvrB lexA) were also about 3-fold less than that of wild-type strain. This observation is in striking contrast to the earlier results with AO and 500-nm light in which strains WP10 and Bs-1 yielded mutation rates that were 12-fold and 5-fold greater, respectively, than the wild-type response. The relatively large decrease in mutation rate when the uvrA endonuclease was absent together with structural considerations in the binding of AO to DNA lead us to propose that the major lesions leading to mutations produced by 460-nm light in the presence of AO may be true DNa single-strand breaks and occur before DNA replication.