Cloning and sequencing of cDNA encoding rat GTP cyclohydrolase I. The first enzyme of the tetrahydrobiopterin biosynthetic pathway.

A full-length cDNA clone for GTP cyclohydrolase I, the first enzyme of the tetrahydrobiopterin biosynthetic pathway, was isolated and characterized. Synthetic oligonucleotides, constructed according to selected amino acid sequences of purified GTP cyclohydrolase I, were used to screen a rat liver cDNA library. Four clones were isolated, and the length of the longest cDNA insert was 1024 base pairs. The identity of the cDNA was confirmed by amino acid sequence data for eight fragments obtained by lysyl endopeptidase digestion of the purified protein. The coding region encoded a protein of 241 amino acid residues, but the NH2 terminus of the protein contained 11 additional amino acid residues not present in the purified protein. RNA blot analysis showed a single mRNA species of 1.2 kilobases in rat liver. A characteristic feature of the deduced amino acid sequence of GTP cyclohydrolase I was the presence of sequences similar to those proposed for the phosphorylation sites for casein kinase II and growth-associated histone H1 kinase. Furthermore, significant similarity was found to the highly conserved sequences of dihydrofolate reductases, which are known to be involved in the binding of the pterin group of dihydrofolate to the reductases. This region in GTP cyclohydrolase I may be assigned to the binding site of tetrahydrobiopterin, one of the inhibitors of this enzyme.