Investigation of genomic island 2 deciphers the evolution of a genus Brucella

HGT acquisition of genomic island 2 (GI-2) into the Brucella genome endowed these organisms with two transglycosylases which function in the final steps of the O-polysaccharide (OPS) polymerization. Here, the investigation of the brucellaphage Pr genome revealed sequence similarities to GI-2 in two DNA regions, the Chelativorans sp. BNC1 and the Ochrobactrum anthropi ATCC 49188 DNAs, respectively. This led the investigation of the published GI-2 elements, using the phage attB and attP sequences as hallmarks of the integration site. The study identified a Mesorhizobium loti integrative, conjugative element (ICE) into which GI-2 integrated. A similar ICE, which nevertheless lacks GI-2, and shows characteristic chromosomal architectural differences between the taxa, was found in the genomes of the Ochrobactrum sister clade and the alphaproteobacterial Chelativorans sp. BNC1 strains. This proves that Brucella and Ochrobactrum diverged at the genus level while being in the state of a monopartite genome bearing organism.