Catalyzes the formation of 6,7-dimethyl-8-ribityllumazine by condensation of 5-amino-6-(D-ribitylamino)uracil with 3,4-dihydroxy-2-butanone 4-phosphate. This is the penultimate step in the biosynthesis of riboflavin. (2S)-2-hydroxy-3-oxobutyl phosphate + 5-amino-6-(D-ribitylamino)uracil = 6,7-dimethyl-8-(1-D-ribityl)lumazine + H(+) + 2 H2O + phosphate Cofactor biosynthesis; riboflavin biosynthesis; riboflavin from 2-hydroxy-3-oxobutyl phosphate and 5-amino-6-(D-ribitylamino)uracil: step 1/2. Homopentamer. The two ribH genes may be differentially expressed during the Brucella infection cycle. Brucella would use RibH1 for flavin biosynthesis during the extracellular phase and RibH2 during intracellular growth. Belongs to the DMRL synthase family.