The lethal and mutagenic effects of ultraviolet light (u.v.), ethyl methanesulphonate (EMS), methyl methanesulphonate (MMS), -methyl-'-nitro--nitrosoguanidine (MNNG) and mitomycin C on the unicellular cyanobacterium have been investigated. Mitomycin C and EMS did not significantly increase the frequency of streptomycin-resistant mutants above the spontaneous level. MNNG and MMS produced increases of 10 to 30 times the spontaneous frequency. Contrary to previous reports on mutation induction in cyanobacteria, u.v. was the most effective mutagen, increasing the spontaneous mutation frequency 4000-fold. However, since photoreactivating light eliminated most of this mutagenic response it was necessary to rigorously exclude light of wavelengths < 550 nm during post-irradiation growth. Caffeine failed to increase the mutation frequency of u.v.-irradiated cultures, but cells were more sensitive to u.v. killing in the presence of caffeine.


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