Molecular characterization of a Brucella species large DNA fragment deleted in Brucella abortus strains: Evidence for a locus involved in the synthesis of a polysaccharide

A Brucella melitensis 16M DNA fragment of 17,119 bp, which contains a large region deleted in B. abortus strains and DNA flanking one side of the dele tion, has been characterized. In addition to the previously identified omp3 1 gene, 14 hypothetical genes have been identified in the B. melitensis fra gment, most of them showing homology to genes involved in the synthesis of a polysaccharide. Considering that 10 of the 15 genes are missing in B. abo rtus and that all the polysaccharides described in the Brucella genus (lipo polysaccharide, native hapten, and polysaccharide Il) have been detected in all the species, it seems likely that the genes described here might be pa rt of a cluster for the synthesis of a novel Brucella polysaccharide. Sever al polysaccharides have been identified as important virulence factors, and the discovery of a novel polysaccharide in the brucellae which is probably not synthesized in B. abortus might be interesting for a better understand ing of the pathogenicity and host preference differences observed between t he Brucella species. However, the possibility that the genes described in t his paper no longer encode the synthesis of a polysaccharide cannot be excl uded. Brucellae belong to the alpha-2 subdivision of the class Proteobacter ia, which includes other microorganisms living in association with eucaryot ic cells, some of them synthesizing extracellular polysaccharides involved in the interaction with the host cell, The genes described in this paper mi ght be a remnant of the common ancestor of the alpha-2 subdivision of the c lass Proteobacteria, and the brucellae might have lost such extracellular p olysaccharide during evolution if it was not necessary for survival or for establishment of the infectious process. Nevertheless, further studies are necessary to identify the entire DNA fragment missing in B. abortus strains and to elucidate the mechanism responsible for such deletion, since only 9 ,948 bp of the deletion was present in the sequenced B. melitensis DNA frag ment.