Catalyzes the dismutation of two molecules of 6,7-dimethyl-8-ribityllumazine, resulting in the formation of riboflavin and 5-amino-6-(D-ribitylamino)uracil. 2 6,7-dimethyl-8-(1-D-ribityl)lumazine + H(+) = 5-amino-6-(D-ribitylamino)uracil + riboflavin Cofactor biosynthesis; riboflavin biosynthesis; riboflavin from 2-hydroxy-3-oxobutyl phosphate and 5-amino-6-(D-ribitylamino)uracil: step 2/2. Homotrimer. Repressed by iron.