The 18-kDa cytoplasmic protein of Brucella species - an antigen useful fordiagnosis - is a lumazine synthase

Previous studies have shown that the detection of antibodies to an 18-kDa c ytoplasmic protein of Brucella spp. is useful for the diagnosis of human an d animal brucellosis, This protein has now been expressed in recombinant fo rm in Escherichia coli. The recombinant protein is soluble only under reduc ing conditions, but alkylation with iodoacetamide renders it soluble in non -reducing media. As shown by gel exclusion chromatography, this soluble for m arranges in pentamers of 90 kDa. The reactivity of human and animal sera against the recombinant protein was similar to that found with the native p rotein present in brucella cytoplasmic fraction, suggesting that the recomb inant protein is correctly folded. The protein has low but significant homo logy (30%) with lumazine synthases involved in bacterial riboflavin biosynt hesis, which also arrange as pentamers. Biological tests on the crude extra ct of the recombinant bacteria and on the purified recombinant protein show ed that the biological activity of the Brucella spp, 18-kDa protein is that of lumazine synthase, Preliminary crystallographic analysis showed that th e Brucella spp. lumazine synthase arranges in icosahedric capsids similar t o those formed by the lumazine synthases of other bacteria. The high immuno genicity of this protein, potentially useful for the design of acellular va ccines, could be explained by this polymeric arrangement.