Involved in riboflavin biosynthesis. Catalyzes the conversion of GTP to 2,5-diamino-6-ribosylamino-4(3H)-pyrimidinone 5'-phosphate (DARP), formate and pyrophosphate. RIBA2 and RIBA3 together are not able to complement the loss of function of RIBA1. GTP + 4 H2O = 2,5-diamino-6-hydroxy-4-(5-phosphoribosylamino)-pyrimidine + formate + 3 H(+) + 2 phosphate Binds 1 zinc ion per subunit. Cofactor biosynthesis; riboflavin biosynthesis; 5-amino-6-(D-ribitylamino)uracil from GTP: step 1/4. A number of isoforms are produced. Expressed in leaves, shoots, roots, flowers and siliques. In the N-terminal section; belongs to the DHBP synthase family. In the C-terminal section; belongs to the GTP cyclohydrolase II family. The substrate-binding sites for the inactive DHBP synthase activity are conserved while several cofactor-binding sites are lost.