Isolation and properties of a Euglena mutant resistant to the killing and bleaching effects of N-methyl-N′-nitro-N-nitrosoguanidine and some other agents

Euglena gracilis cells were exposed to a high concentration of N-methyl-N′-nitro-N-nitrosoguanidine (NG) and then grown on CO2 as the sole carbon source. Among the survivors were cells which were 2- to 4-fold resistant to both the killing and bleaching effects of NG. Growth of a typical resistant mutant is inhibited for a shorter time by modest concentrations of NG than is growth of the sensitive, parental Strain z. Likewise, nucleic acid synthesis in the resistant mutant is affected much less strongly than nucleic acid synthesis in Strain z. These results are discussed in terms of the possibility that the NG-resistant cells are better able to repair NG-induced damage.In the resistant mutants, chloroplast-forming ability is cross-resistant to nitrofurantoin but not to ultraviolet light, while colony-forming ability is resistant to both these agents, suggesting that colony-forming ability and chloroplast-forming ability have become resistant through independent mutations. In addition, colony-forming ability is resistant to γ-radiation and to nitrogen mustard; agents which do not cause bleaching. Thus the colony-forming ability of NG-resistant Euglena shows a cross-resistance pattern similar to bacterial mutants selected for resistance to radiation or a radiomimetic chemical.