The cps gene cluster of Salmonella strain LT2 includes a second mannose pathway: sequence of two genes and relationship to genes in the rfb gene cluster

Abstract

We report the presence in Salmonella enterica strain LT2 (serovar thyphimurium) of duplicate genes for two steps in the synthesis of GDP-mannose. The previously known genes, rfbK (phosphomannomutase) and rfbM (mannose-l-phosphate guanyltransferase), are part of the gene cluster for the O antigen. The two new genes, cpsB and cpsG, respectively, are thought to be part of the gene cluster for the M antigen capsular polysaccharide, present in many Enterobacteriaceae. The two genes have been sequenced and have a GC content of 0.61, suggesting an origin outside of Salmonella. Comparison of the inferred protein sequences for cpsB and rfbM shows 57% identity of amino acids whereas for cpsG and rfbK there is only 19% identity. It is suggested that the greater divergence between cpsG and rfbK may be due to a period of accelerated evolution, perhaps precipitated by transfer of the genes from another species.