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 Is activated by sulfite ions. Optimum pH is 7.4. Cofactor biosynthesis; riboflavin biosynthesis; riboflavin from 2-hydroxy-3-oxobutyl phosphate and 5-amino-6-(D-ribitylamino)uracil: step 2/2. Homotrimer. Can interact with 6,7-dimethyl-8-ribityllumazine synthase, forming a lumazine synthase/riboflavin synthase complex, also designated as 'heavy riboflavin synthase complex', which consists of a trimer of riboflavin synthase enclosed within an icosahedral structure composed of 60 subunits of 6,7-dimethyl-8-ribityllumazine synthase.