Considering the importance of carbohydrate moieties on infectivity and host mimicry, there is a need to better understand the biosynthetic pathways of these unusual sugars in order to identify key targets involved in bacterial pathogenesis.Escherichia coli strain serogroup O119 is commonly associated with septicemia or diarrhea in calves and pose a significant threat to the cattle industry worldwide.2-acetamido-2,3-dideoxy-3-formamido-L-rhamnose (L-RhaNAc3Fo), an uncommon sugar, only found in Escherichia coli O119 O antigen.The biosynthetic pathway of this sugar has never been fully characterized.In our previous study, four genes (off 7, orf5, orf6 and orf4) from the E.eoli O119 O antigen gene cluster were proposed to encode the enzymes for the synthesis of dUDP-L-RhaNAc3Fo (Fig.1).