Complementation of the fol2 deletion in Saccharomyces cerevisiae by human and Escherichia coli genes encoding GTP cyclohydrolase I

Saccharomyces cerevisiae is so far the only organism where a knock-out mutant in the gene encoding GTP cyclohydrolase I (FOL2) has been obtained. GTP cyclohydrolase I controls the de novo biosynthetic pathway of tetrahydrobiopterin and folic acid. Since deletion of yeast FOL2 leads to a recessive auxotrophy for folinic acid, we used a yeast fol2Delta mutant for an in vivo functional assay of heterologous GTP cyclohydrolases I. We show that the GTP cyclohydrolase I, encoded either by the E. coli folE gene or by the human cDNA, complements the yeast fol2Delta mutation by restoring folate prototrophy. Furthermore the folE-3x allele of the E. coli gene, carrying three base substitutions, failed to complement the yeast fol2Delta defect. This allele behaved as a negative semidominant to the wild type folE and, when overexpressed, completely abolished complementation of fol2Delta by folE. Thus, the yeast fol2 null mutant is a suitable system to characterize mutations in genes encoding GTP cyclohydrolase I.