Ribonucleotide reductase is responsible for providing the
deoxyribonucleotide precursors for
DNA synthesis. In most species the
enzyme consists of a large and a small subunit, both of which are required for activity. In mammalian cells, the small subunit is the site of action of several
antitumor agents, including
hydroxyurea and
4-methyl-5-amino-1-formylisoquinoline thiosemicarbazone (
MAIQ). The
mRNA levels for the small subunit of
ribonucleotide reductase (RNR2) and sensitivity to
hydroxyurea and
MAIQ were determined in four strains of the yeast, Saccharomyces cerevisiae. Two strains exhibited significantly different sensitivities to both
hydroxyurea and
MAIQ, which closely correlated with differences in the levels of RNR2
mRNA. These results are consistent with recent observations with mammalian cells in culture, and indicate that a common mechanism of resistance to
hydroxyurea and related drugs occurs through the elevation in
ribonucleotide reductase message levels. A transplason mutagenized strain with marked structural modifications in RNR2
DNA and
mRNA showed an extreme
hypersensitivity to
hydroxyurea but not to
MAIQ, providing evidence that the two drugs do not inhibit the RNR2 subunit by the same mechanism. In addition, a yeast strain isolated for low but reproducible resistance to
MAIQ exhibited a sensitivity to
hydroxyurea similar to the parental wild-type strain, supporting the idea that the two drugs inhibit the activity of RNR2 by unique mechanisms. These yeast strains provide a useful approach for further studies into the regulation of eucaryotic
ribonucleotide reduction and drug resistance mechanism involving a key rate-limiting step in
DNA synthesis.