The mutant mouse lymphoma cell line (L5178YAII), resistant to X-rays, ultraviolet light and alkylating agents, was reinvestigated in an attempt to establish the nature of the mutation. These cells were compared with P388 mouse lymphoma cells, which exhibit normal sensitivity to these mutagens. A series of studies was conducted to compare DNA alkylation and strand breakage with cell survival after exposure of the two cell lines to methylmethane sulphonate. It was found that neither the degree of alkylation nor the removal of the common alkylation products was correlated with the different sensitivities observed in these cell lines. A correlation was established between cell killing and the production of long-lived strand breaks. P388 cells were found to accumulate twice as many long-lived strand breaks compared to L5178YAII cells, at equal levels of alkylation. This suggested that long-lived strand breaks were the major toxic lesions. Further experiments indicated that these long-lived strand breaks were produced by a process consistent with excision repair. Evidence is also presented that indicates that the mutation in L5178YAII cells that is responsible for their resistance may occur in ligase activity or its associated ADP-ribosyl transferase system.

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