Genetic organisation and evolution of Yersinia pseudotuberculosis 3,6-dideoxyhexose biosynthetic genes.

3,6-dideoxyhexose (DDH) sugars occur in some of the O antigens of Salmonella enterica and Yersinia pseudotuberculosis, but are otherwise rarely found in nature. Y. pseudotuberculosis DDH biosynthetic genes rfbS (encoding CDP-paratose synthetase) and rfbE (encoding CDP-tyvelose epimerase) were amplified and cloned, and their sequences determined. Comparisons with the equivalent genes of S. enterica show that the genetic arrangement of DDH genes is very similar; however, in Y. pseudotuberculosis there is no suggestion that paratose producing strains are derived from tyvelose-producing strains by inactivation of rfbE, which is the case in S. enterica. The previously determined DNA sequence of the rfb region of an abequose-producing strain was re-examined. It contains the remnants of an insertion sequence (IS) adjacent to a truncated and non-functional rfbE gene. This suggests that the IS was involved in recombination events contributing to O-antigen antigenic diversity in Y. pseudotuberculosis.