MutT protein of Escherichia coli prevents the occurrence of A:T --> C:G 
transversion by hydrolyzing an oxidized form of dGTP, 8-oxo-7, 
8-dihydro-2'-deoxyguanosine 5'-triphosphate (8-oxo-dGTP), which is produced by 
active oxygen species. In a search for mutT-related genes, we found that the 
ribA gene, encoding GTP cyclohydrolase II, is able to reduce the increased level 
of mutation frequency of the mutT strain to almost the normal level, provided 
that the gene product is overproduced. Purified preparations of Escherichia coli 
GTP cyclohydrolase II protein as well as the histidine hexamer-tagged 
recombinant GTP cyclohydrolase II protein efficiently hydrolyze 8-oxo-dGTP and 
8-oxo-GTP, producing 8-oxo-dGMP and 8-oxo-GMP, respectively. dGTP was not 
hydrolyzed by these preparations. GTP cyclohydrolase II catalyzes conversion of 
GTP to 2, 5-diamino-6-hydroxy-4-(5-phosphoribosylamino)-pyrimidine, which 
constitutes the first step for riboflavin synthesis. The Km values for the three 
types of guanine nucleotides, GTP, 8-oxo-GTP, and 8-oxo-dGTP, were almost the 
same. In the mutT- background, ribA- cells showed higher spontaneous mutation 
frequencies as compared with that of ribA+ cells. Thus, GTP cyclohydrolase II, 
the ribA gene product, has a potential to protect genetic material from the 
untoward effects of endogenous oxygen radicals.