5-amino-6-(D-ribitylamino)uracil + L-tyrosine + S-adenosyl-L-methionine = 5-amino-5-(4-hydroxybenzyl)-6-(D-ribitylimino)-5,6-dihydrouracil + 2-iminoacetate + 5'-deoxyadenosine + L-methionine + H(+) — reaction