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

In the presence of acridine orange (AO) and monochromatic 500-nm light, the recombination-deficient strain of Escherichia coli, WP10 (recA), showed a 15-fold increase in mutation rate over the wild-type (WP2) strain. Under the same conditions, strain Bs--1 (uvrB lexA lon) showed a 5-fold increase in mutation rate over strain WP2. In contrast, the endonuclease-deficient, strain, WP2s (uvrA), showed a lower AO-500 nm mutation rate than wild-type. The extremely high mutation rate of the recA strain cannot be due to error-prone inducible SOS repair since the inducible recA + function is absent. Repair of the AO-500 nm-induced lesions is likely due to a recA+-dependent, error-free, recombination process. It is concluded that the high mutation rates with AO-500 nm light obtained in chemostat cultures of recA and lexA strains occur as a consequence of errors during semi-conservative DNA replication in the presence of unrepaired DNA lesions.