SUMMARY: Chemically induced mutation in the cyanobacterium was studied using resistance to the pyrimidine analogue 5'-fluorocytosine as a genetic marker which can be selected positively. Cytosine is metabolized through uracil and the UMP pyrophosphorylase ‘salvage’ pathway in this photoautotroph, as it is in enteric bacteria. Treatment with various concentrations of -methyl-'-nitro--nitrosoguanidine (MNNG) gave the highest frequencies of 5FC-resistant mutants when lethality approximated 99%, irrespective of the exposure time or mutagen concentration. The pH of the incubation medium strongly influenced mutation; exposure to MNNG at pH 6.0 yielded 13-fold higher frequencies of mutants than at pH 7.5. The greatest frequency of resistant cells was found after cultures had undergone six or more doublings following mutagenesis. The mutation frequencies obtained by treatment with MNNG were approximately 4- and 25-fold higher than those after exposure under empirically defined conditions to diethyl sulphate or nitrous acid, respectively, and 1.4×10-fold higher than the frequency of spontaneous mutation. Neither chloramphenicol-inhibited nor caffeine-sensitive systems capable of repairing MNNG-induced DNA damage were observed.


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