Loss of hydrazine-induced mutability in wild-type and excision-repair-defective yeast during post-treatment inhibition ofcell division.

The fate of presumed premutational DNA lesions induced by hydrazine was studied under a variety of post-treatment conditions in wild-type and in excision repair-defective (rad2-1) strains of Saccharomyces cerevisiae. In all strains the full extent of hydrazine-induced, forward mutability from CAN1 to can1 (canavanine resistance) was dependent upon post-treatment cell division in mutagen-free synthetic or complex growth medium before plating on canavanine-containing selective agar and could be blocked by inhibitors of DNA synthesis (hydroxyurea) or protein synthesis (cycloheximide) contained in the growth medium. Following the growth-inhibitory period, cells were permitted to grow in fresh medium lacking inhibitors to determine the level of induced mutation remaining. Nearly all induced mutability was lost after a one-day growth inhibition, compared with mutagen-treated control samples subsequently grown twice in medium lacking inhibitor. In the wild type, half the induced mutability was lost after 3 h. The data suggest that premutational DNA lesions induced by hydrazine were removed, or possibly rendered non-mutagenic, by some error-free repair process that acted before mutation fixation by base mispairing during DNA replication. Since rad2-1 and RAD strains both exhibited loss of mutability, this process is not dependent upon the activity of an intact pyrimidine dimer excision-repair system.