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 inhibited by riboflavin. Product inhibition may be the major mechanism by which RS regulates its enzymatic activity in vivo. Cofactor biosynthesis; riboflavin biosynthesis; riboflavin from 2-hydroxy-3-oxobutyl phosphate and 5-amino-6-(D-ribitylamino)uracil: step 2/2. Homotrimer. Each monomer can bind two substrate molecules, yet there is only one active site for the whole trimeric enzyme, which is located at the interface between two neighboring chains and formed by the N-terminal barrel from chain A and the C-terminal barrel from chain B.