Inhibition and Enhancement of Phleomycin-induced DNA Breakdown by Aromatic Tricyclic Compounds

Cationic aromatic tricyclic compounds including triphenylmethane dyes, phenazines, phenoxazines, acridines, phenothiazines, phenanthridinium compounds, anthracenes and xanthene dyes, which amplify cell killing in phleomycin-treated Escherichia coli B cells also modified phleomycin-induced breakdown of DNA to acid-soluble fragments. A plot of DNA breakdown as a function of concentration was bell-shaped for each of the active compounds, i.e. as the concentration increased, DNA breakdown was enhanced initially, but above a certain concentration, the proportion of DNA degraded declined, often to zero. One of the compounds, acriflavine, when tested also inhibited DNA breakdown following ultraviolet irradiation. A study, by sedimentation methods, of DNA single-strand breakage in phleomycin-treated E. coli cells, using 3 representative compounds, Crystal Violet, 3,6-diaminoacridine and Methylene Blue, revealed a consistent increase in DNA strand breaks as concentration of compound increased. In similar experiments with ethidium bromide the breakage yield/concentration curve exhibited a maximum. In general, however, it seems that the inhibition of DNA-breakdown observed at higher concentrations of these amplifying compounds is not explicable by an effect on the primary breakage event, but is due to suppression of exonucleolytic activity in the cells.