The addition of 6-azauracil to the growth medium causes a strong reduction of the GTP level in the nucleotide pool of Saccharomyces cerevisiae. In-vitro experiments show a strong inhibition of IMP dehydrogenase activity by 6-azaUMP explaining the preceding effect. PPR2 mutants, previously characterized by an increased sensitivity to 6-azauracil compared to the wild-type, are specifically susceptible to the lowering of the GTP pool, and are able to grow in presence of 6-azauracil when guanine is added to the medium.