Molecular evolution of the GDP-mannose pathway genes (manB and manC) in Salmonella enterica

The evolutionary history of the GDP-mannose pathway in Salmonella enterica was studied via sequencing manB and manC genes from 13 representative strains for O antigens containing mannose and/or sugar derivatives of GDP-D-mannose. In addition, colanic acid (CA) manB and manC genes were sequenced from selected strains, as the basis for a detailed comparison. Interestingly, including the eight previously characterized O antigen gene clusters, 12 of the 21 S. enterica strains studied in total (each representing a different O antigen structure) possess a manB gene which displays DNA identity, ranging from 93 to 99%, to the CA manB gene of S. enterica LT2. Furthermore, the CA-like manB genes (as well as the CA manB and manC genes) display subspecies specificity, and the CA and CA-like manB genes (for individual strains) appear to be evolving in concert via gene conversion events. In comparison, the manC genes were generally not CA-like, a situation also apparent in Escherichia coli, and therefore most strongly reflected the evolutionary history of the S. enterica O antigen GDP-mannose pathway. It appears that, in relatively recent times, gene capture from a distant source has occurred infrequently, and that groups of manB and manC genes have been maintained and are continuing to evolve within S. enterica and more closely related species.