Divergence in macromolecular assembly: X-ray crystallographic structure analysis of lumazine synthase from Brucella abortus.

We have determined the three-dimensional structure of 6, 7-dimethyl-8-ribityllumazine synthase (lumazine synthase) from Brucella abortus, the infectious organism of the disease brucellosis in animals. This enzyme catalyses the formation of 6, 7-dimethyl-8-ribityllumazine, the penultimate product in the synthesis of riboflavin. The three-dimensional X-ray crystal structure of the enzyme from B. abortus has been solved and refined at 2.7 A resolution to a final R-value of 0.18 (R(free)=0.23). The macromolecular assembly of the enzyme differs from that of the enzyme from Bacillus subtilis, the only other lumazine synthase structure known. While the protein from B. subtilis assembles into a 60 subunit icosahedral capsid built from 12 pentameric units, the enzyme from B. abortus is pentameric in its crystalline form. Nonetheless, the active sites of the two enzymes are virtually identical indicating inhibitors to theses enzymes could be effective pharmaceuticals across a broad species range. Furthermore, we compare the structures of the enzyme from B. subtilis and B. abortus and describe the C teminus structure which accounts for the differences in quaternary structure.