A synergistic effect of ultraviolet light and ethidium bromide on petite induction in yeast

Mitochondrial genetic damage induced in yeast by a photoactivated furocoumarin in combination with ethidium bromide or ultraviolet light

Ethidium bromide (EB) and ultraviolet light (UV) in combination are known to produce a synergistic induction of "petite" mutants in yeast. Two other agents were combined with EB, 3-Carbethoxypsoralene (3 CPs) activated by 365 nm light or gamma rays. EB in combination with 3 CPs also resulted in an enhanced production of "petite" mutants. After the photoaddition of 3 CPs in exponential phase cells, recovery of the "petite" mutation during dark liquid holding was inhibited by the presence of EB producing an enhanced number of "petite" mutants. The behavior of mitochondrial antibiotic resistance markers after individual and combined treatments with EB and 3 CPs indicates a random loss of markers after EB and a preferential loss of a certain region for the 3 CPs photoaddition. The combination of the two agents leads to an additivity of total drug marker losses rather than a synergistic loss. The combination of EB with gamma rays produced no enhancement in "petite" induction. A combination of UV and 3 CPs showed a synergistic interaction for "petite" induction. These results indicate that the three agents, EB, UV and 3 CPs photoaddition may share a common repair step for mitochondrial lesions.

A protective effect of caffeine on the ethidium induced petite mutation in yeast

A prerequisite for petite induction by ethidium bromide (EB) is an initial covalent attachment of the drug to cytoplasmic DNA. This DNA modification is thought to initiate repair processes. The repair inhibitor, caffeine, provided a protective effect against the ethidium induced petite mutation at caffeine concentrations known to inhibit the repair of UV damage in cytoplasmic DNA (Fig. 1). Mitochondrial DNA isolated from yeast exposed to EB in vivo was not as degraded in the presence of both drugs as with EB alone (Fig. 2).