Biosynthesis of riboflavin in Archaea -: 6,7-Dimethyl-8-ribityllumazine synthase of Methanococcus jannaschii

Heterologous expression of the putative open reading frame MJ0303 of Methanococcus jannaschii provided a recombinant protein catalysing the formation of the riboflavin precursor, 6,7-dimethyl-8-ribityllumazine, by condensation of 5-amino-6-ribitylamino-2,4(1H ,3H )-pyrimidinedione and 3,4-dihydroxy-2-butanone 4-phosphate. Steady state kinetic analysis at 37 degreesC and pH 7.0 indicated a catalytic rate of 11 nmol.mg(-1) .min(-1) ; K (m) values for 5-amino-6-ribitylamino-2,4(1H ,3H )-pyrimidinedione and 3,4-dihydroxybutanone 4-phosphate were 12.5 and 52 mum, respectively. The enzyme sediments at an apparent velocity of about 12 S. Sedimentation equilibrium analysis indicated a molecular mass around 1 MDa but was hampered by nonideal solute behaviour. Negative-stained electron micrographs showed predominantly spherical particles with a diameter of about 150 Angstrom. The data suggest that the enzyme from M . jannaschii can form capsids with icosahedral 532 symmetry consisting of 60 subunits.