Summary: strain M28, which has been used widely in genetic studies, was found to carry a radiation-sensitive mutation. This allele, termed was recessive in heterozygous diploids and mapped in linkage group III. Complementation analysis and survival studies on strains carrying suggested that this allele defines a new radiation-sensitive locus in and this locus has been designated strains were moderately sensitive to ultraviolet light ( 90 J m) and slightly sensitive to Cs gamma rays ( 255 krad). strains also exhibited increased sensitivity to killing by -methyl--nitro--nitrosoguanidine but not by other alkylating agents such as ethyl methanesulphonate or methyl methanesulphonate. The frequency of spontaneous methanol-resistant () mutants was approximately the same in cultures of and strains. However, when amoebae of these strains were irradiated with ultraviolet light, the frequency of induced mutants was significantly lower in cultures of the strain. Furthermore, when amoebae of wild-type strain NC4 were plated in the presence of caffeine after ultraviolet-irradiation, the survival curves were very similar to the curves obtained for amoebae of strains in the presence or in the absence of caffeine. These results suggest that the mutation and caffeine interfere with an error-prone DNA repair pathway in


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