The enteric bacterium Salmontlla typhimurium is able to metabolize 1.2-propanediol as a source of energy under anaerobic conditions. The first enzyme in this fermentation pathway, propanediol dehydratase, is dependent upon the coenzyme adenosylcobalamin (coenzyme B12) for its activity. The genes that encode the enzymes in this pathway are organized into an operon designated pdu (propandiol utilization). A 6.3-kbp H indlll fragment of the pdu region in S. typhimurium chromosome has been characterized. A restriction map of this fragment has been constructed using seven different restriction endonucleases to aid in further manipulations of the DNA. The Hindlll fragment has been expressed using the T7 RNA polymerase/promoter (two-plasmid) system. Four protein bands with estimated molecular masses of 60, 29, 26, and 19 kDa have been observed using SDS-polyacrylamide gel electrophoresis. Deletions of different sizes within the H in dill fragment have been generated by resbiction endonuclease digestions. Each of the deleted fragments was expressed using the 1i protein expression system. From the sizes of the proteins expressed from each deleted fragment. the order of the genes within the 6.3-kbp Hindlll fragment was deduced.