Catalyzes the conversion of D-ribulose 5-phosphate to formate and 3,4-dihydroxy-2-butanone 4-phosphate. 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. In the N-terminal section; belongs to the DHBP synthase family. In the C-terminal section; belongs to the GTP cyclohydrolase II family.