Modification of radiation effects on E. coli B/r and a radiosensitive mutant Bs-1 by membrane-binding drugs

Mutagenic and cytotoxic effects of oxygen free radicals generated by methylviologen (paraquat) on Escherichia coli with different DNA-repair capacities

Investigations were carried out to examine the mutagenic and cytotoxic effects of oxygen free radicals on E. coli. E. coli B strains with different DNA-repair capacities were exposed to methyl viologen, commonly called paraquat (1,1'-dimethyl-4,4'-bipyridinium dichloride, MV), which has been shown to act as an intracellular generator of superoxide radicals. The results obtained were as follows: The cytotoxicity of MV in E. coli was dioxygen-dependent and due to the extent of intracellular generation of superoxide radicals. Cells containing higher levels of superoxide dismutase were more resistant to the toxic effect of MV. The cytotoxicity of MV was greater in DNA repair-deficient E. coli, Bs-1(exrA uvrB), NG30(recA) and R15(polA), than in DNA-repair-proficient strains (B/r and H/r30) and Hs30 (uvrB). MV was found to be mutagenic to E. coli H/r30 and Hs30 under aerobic conditions. The mutation frequencies to streptomycin resistance and to arginine prototrophy increased with the dose of MV in both strains. However, E. coli NG30 was unmutable by MV. The mutation induction did not occur under anaerobic conditions. The expression of the umu operon in E. coli was induced by MV under aerobic conditions. From these results, it was concluded that superoxide radicals intracellularly generated by MV include DNA damage, which causes cytotoxicity and mutation induction in E. coli, and that DNA damage induced by oxygen radicals is repairable by at least recA, polA and exrA(lexA) gene-controlled mechanisms.

Inhibitory effect of membrane-binding drugs on excision repair of DNA damage in UV-irradiated Escherichia coli

The effects of procaine and lidocaine on DNA-repair processes were investigated in UV-irradiated cells of E. coli with different DNA-repair capacities. The cells were irradiated with various doses of UV and then incubated at 37 degrees C in M9 buffer (liquid-holding) or in EM9 medium in the presence or absence of membrane-binding drugs. The results obtained are as follows. (1) In strains H/r30 (wild-type for DNA repair) and NG30 (recA-), the increase in survival with increase in time of liquid-holding was almost completely inhibited by the addition of procaine and lidocaine. The same trends were observable under conditions of post-irradiation incubation in EM9 medium, more efficiently in recA- strain than in the wild-type strain. (2) The addition of these drugs gave an apparent enhancement of the frequency of UV-induced mutation to arginine prototrophy, corresponding to a decrease in survival. (3) There were negligible effects of the drugs on survival and mutation in the excision-repair-defective strain, Hs30 (uvrB-). (4) The removal of thymine dimers from DNA was actually reduced by the addition of procaine. From these results it is concluded that procaine and lidocaine inhibited excision-repair process in UV-irradiated E. coli cells. Procaine and lidocaine are typical local anesthetics and known to interact with cell membranes causing alterations in the structural and functional organization. Therefore, it is suggested that a disorganization of the membrane structure brought about by the drugs may result in an inhibition of excision repair of DNA damage in E. coli, assuming that at least a component of excision repair is associated with the cell membrane. Possible mechanisms involved in this process are discussed.

Influence of misonidazole on survival of wild-type and radiosensitive mutants of yeast

Survival curves were obtained for haploid and diploid yeasts, Saccharomyces cerevisiae, of a wild-type strain and radiosensitive mutants exposed to gamma-rays in oxygenated and hypoxic conditions both in the absence and in the presence of misonidazole. Misonidazole enhanced the radiosensitivity only of hypoxic cells. A correlation between oxygen and misonidazole sensitization was observed. The data confirm that misonidazole mimics the sensitizing effect of oxygen. The degree of liquid-holding recovery of yeast cells was greater when the cells were irradiated in hypoxic conditions with than without misonidazole. Possible reasons for the observed radiation responses are discussed.