Pathways of ultraviolet mutability in Saccharomyces Cerevisiae. IV. The relation between canavanine toxicity and ultraviolet mutability to canavanine resistance.

Seven umr mutants of Saccharomyces cerevisiae which had reduced capacity for ultraviolet light (UV)-induced forward mutation from CAN1 to can1 were tested for sensitivity to L-canavanine relative to one wild-type UMR strain and one slightly UV-sensitive but phenotypically umr⁺ strain (mutant 306). Relative UV mutation resistance was estimated by dividing the UV fluence needed to yeild a particular induced mutation frequency by that needed to reach the same frequency in the genotypic wild-type strain. The umr5 and umr6 strains were especially sensitive to canavanine growth inhibition, while umr1 was no more sensitive than either wild type; umr2, umr3, umr4, a umr7, and α umr7 were equally sensitive to an intermediate degree. Incubation at 30°C of wildtype cells plated on canavanine-selective agar for increasingly longer times before UV irradiation resulted in decreasing UV mutation frequencies (reduced to 50% in 1.6 h). All umr strains tested in this way lost UV mutability faster than wild type, including mutant 306, umr1 (not sensitive to growth inhibition), and umr6 (very sensitive to growth inhibition). Cells were grown to stationary phase in YEDP growth medium and assayed for arginine and tryptophan transport into the cell. The umr6 strain, which had weak UV mutation resistance but high sensitivity to canavanine growth inhibition, transported arginine and tryptophan at essentially wild-type levels. The umr1 strain, however, which had moderate UV mutation resistance and normal canavanine toxicity, transported both amino acids at rates tenfold higher than wild type. The data suggest that increased canavanine toxicity does not necessarily lead to defective mutability at CAN1, and that mutational deficiency cannot result solely from increased canavanine toxicity. Although exposure to canavanine was shown to block mutation fixation and/or expression, it is suggested that the degree of growth inhibition is not strictly correlated with the degree of mutation resistance.