Catalyzes the condensation of 5-amino-6-(1'-D)-ribityl- amino-2,4(1H,3H)-pyrimidinedione with L-3,4-dihydrohy-2-butanone- 4-phosphate yielding 6,7-dimethyl-8-lumazine. MASFAASQTCFLTTNPTCLKPNSPQKSSTFLPFSAPLSSSSSFPGCGLVHVASNKKNRASFVVTNAVRELEGYVTKAQSFRFAIVVARFNEFVTRRLMEGALDTFKKYSVNEDIDVVWVPGAYELGVTAQALGKSGKYHAIVCLGAVVKGDTSHYDAVVNSASSGVLSAGLNSGVPCVFGVLTCDNMDQAINRAGGKAGNKGAESALTAIEMASLFEHHLKA 222 RISB_SPIOL 6,7-dimethyl-8-ribityllumazine synthase, chloroplastic