Previous studies have shown that the detection of antibodies to an 18-kDa cytoplasmic protein of spp. is useful for the diagnosis of human and animal brucellosis. This protein has now been expressed in recombinant form in . The recombinant protein is soluble only under reducing conditions, but alkylation with iodo-acetamide renders it soluble in non-reducing media. As shown by gel exclusion chromatography, this soluble form 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 protein present in brucella cytoplasmic fraction, suggesting that the recombinant protein is correctly folded. The protein has low but significant homology (30%) with lumazine synthases involved in bacterial riboflavin biosynthesis, which also arrange as pentamers. Biological tests on the crude extract of the recombinant bacteria and on the purified recombinant protein showed that the biological activity of the spp. 18-kDa protein is that of lumazine synthase. Preliminary crystallographic analysis showed that the spp. lumazine synthase arranges in icosahedric capsids similar to those formed by the lumazine synthases of other bacteria. The high immunogenicity of this protein, potentially useful for the design of acellular vaccines, could be explained by this polymeric arrangement.


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