Saccharomyces cerevisiae transcription elongation mutants are defective inPUR5 induction in response to nucleotide depletion

IMP dehydrogenase (IMPDH) is the rate-limiting enzyme in the de novo synthe sis of guanine nucleotides. It is a target of therapeutically useful drugs and is implicated in the regulation of cell growth rate. In the yeast Sacch aromyces cerevisiae, mutations in components of the RNA polymerase II (Pol II) transcription elongation machinery confer increased sensitivity to a dr ug that inhibits IMPDH, 6-azauracil (6AU), by a mechanism that is poorly un derstood. This phenotype is thought to reflect the need for an optimally fu nctioning transcription machinery under conditions of lowered intracellular GTP levels. Here we show that in response to the application of IMPDH inhi bitors such as 6AU, wild-type yeast strains induce transcription of PUR5, o ne of four genes encoding IMPDH-related enzymes. Yeast elongation mutants s ensitive to 6AU, such as those with a disrupted gene encoding elongation fa ctor SII or those containing amino acid substitutions in Pol II subunits, a re defective in PUR5 induction. The inability to fully induce PUR5 correlat es with mutations that effect transcription elongation since 6AU-sensitive strains deleted for genes not related to transcription elongation are compe tent to induce PUR5. DNA encompassing the PUR5 promoter and 5' untranslated region supports 6AU induction of a luciferase reporter gene in wild-type c ells. Thus, yeast sense and respond to nucleotide depletion via a mechanism of transcriptional induction that restores nucleotides to levels required for normal growth. An optimally functioning elongation machinery is critica l for this response.