Involved in riboflavin biosynthesis. Catalyzes the conversion of D-ribulose 5-phosphate to formate and 3,4-dihydroxy-2-butanone 4-phosphate. RIBA2 and RIBA3 together are not able to complement the loss of function of RIBA1. D-ribulose 5-phosphate = (2S)-2-hydroxy-3-oxobutyl phosphate + formate + H(+) Binds 2 divalent metal cations per subunit. Magnesium or manganese. Cofactor biosynthesis; riboflavin biosynthesis; 2-hydroxy-3-oxobutyl phosphate from D-ribulose 5-phosphate: step 1/1. 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 GTP cyclohydrolase-2 activity are conserved while several cofactor-binding sites are lost.