The gua1 gene encoding inosine monophosphate dehydrogenase (IMPDH), which catalyses the first step in de novo biosynthesis of guanosine monophosphate (GMP), was cloned in the yeast Schizosaccharomyces pombe by functional complementation of a gua1ura4-D18 mutant strain from a S. pombe DNA genomic library. Complementation analysis revealed a 1.2 kb fragment which segregation analysis confirmed did not code for a suppressor gene. Only 446 nucleotides of the gua1 gene encoding the IMPDH C-terminal residues were found within this 1.2 kb sequence (GenBank, AJ293460). The comparison of this wild-type fragment with the same fragment from the gua1ura4-D18 mutant revealed that there was a point mutation at position 1261 (guanine -> adenine) from the 5' end, corresponding to the amino acid residue 421 (glycine -> serine) of the enzyme. Dot and Northern analyses showed that the gua1 gene was expressed in transformants as well as in the wild-type and the gua1ura4-D18 mutant, but enzyme activity was only detected in wild-type and transformant cells. It seems likely that a 446 by fragment from the 3' end of the gua1 gene abolished the point mutation in the mutant strain, suggesting that this fragment participates in the sequences encoding the active domain of IMPDH in S. pombe.