GTP cyclohydrolase I (encoded by gene folE) is the first Zn(2+)-dependent enzyme of the de novo tetrahydrofolate biosynthetic pathway. It presents in bacteria, fungi, and plants. It catalyzes the conversion of GTP to 7,8-dihydroneopterin triphosphate. The 2.1A resolution structure of GTP cyclohydrolase I from Yersinia pestis CO92 is a pentomer in the asymmetric unit. The structure contains, in the active site, five GTP molecules and Ca(2+) ions probably from crystallization in place of Zn(2+) determined by atomic distances and fluorescence spectrum.