The Snf1/AMP-activated kinases are involved in a wide range of stress responses in eukaryotic cells. We discovered a novel role for the Snf1 kinase in the cellular response to genotoxic stress in yeast. snf1 mutants are hypersensitive to hydroxyurea (HU), methyl-methane sulfonate, and cadmium, but they are not sensitive to several other genotoxic agents. HU inhibits ribonucleotide reductase (RNR), and deletion of SNF1 also increased the growth defects of an rnr4 ribonucleotide reductase mutant. The snf1 mutant has a functional checkpoint response to HU insofar as cells arrest division normally and derepress the transcription of RNR genes. The sensitivity of snf1 to HU or to RNR4 deletion may be due to posttranscriptional defects in RNR function or to defects in the repair of, and recovery from, stalled replication forks. The Mig3 repressor was identified as one target of Snf1 in this pathway. Genetic and biochemical analyses suggest that a weak kinase activity is sufficient to confer resistance to HU, whereas a high level of kinase activity is required for optimal growth on carbon sources other than glucose. Quantitative regulation of Snf1 kinase activity may contribute to the specificity of the effector responses that it controls.
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