Characterization of a Brucella species 25-kilobase DNA fragment deleted from Brucella abortus reveals a large gene cluster related to the synthesis of a polysaccharide

In the present study we completed the nucleotide sequence of a Brucella mel itensis 16M DNA fragment deleted from B. abortus that accounts for 25,064 b p and show that the other Brucella spp. contain the entire 25-kb DNA fragme nt. Two short direct repeats of four nucleotides, detected in the B. melite nsis 16M DNA flanking both sides of the fragment deleted from B. abortus, m ight have been involved in the deletion formation by a strand slippage mech anism during replication. In addition to omp31, coding for an immunogenic p rotein located in the Brucella outer membrane, 22 hypothetical genes were i dentified. Most of the proteins that would be encoded by these genes show s ignificant homology with proteins involved in the biosynthesis of polysacha rides from other bacteria, suggesting that they might be involved in the sy nthesis of a Brucella polysaccharide that would be a heteropolymer synthesi zed by a Wzy-dependent pathway. This polysaccharide would not be synthesize d in B. abortus and would be a polysaccharide not identified until present in the genus Brucella, since all of the known polysaccharides are synthesiz ed in all smooth Brucella species. Discovery of a novel polysaccharide not synthesized in B. abortus might be interesting for a better understanding o f the pathogenicity and host preference differences observed between the Br ucella species. However, the possibility that the genes detected in the DNA fragment deleted in B. abortus no longer lead to the synthesis of a polysa ccharide must not be excluded. They might be a remnant of the common ancest or of the alpha-2 subdivision of the class Proteobacteria, with some of its members synthesizing extracellular polysaccharides and, as Brucella spp., living in association with eukaryotic cells.