Analysis of mutants in 1,2-propanediol metabolism in salmonella typhimurium
Under aerobic conditions, the enteric bacterium Salmonella typhimurium utilizes 1,2-propanediol as a sole carbon and energy source if cobalamin is provided as a supplement in the growth medium. Adenosylcobalamin-dependent propanediol dehydratase, encoded by ihepdu operon, is the first enzyme in the pathway that metabolizes propanediol. The pdu operon is located at 44 minutes on the Salmonella genetic map. The defective transposon Mu dK was introduced into the .S". typhimurium chromosome to study the location and fijnction of these genes. The pdu: Mu dK insertions were also located in the pdu genes at 44 minutes on the Salmonella genetic map. None of the Mu dK insertions formed protein fusions, but they all had propanediol-inducible propanediol dehydratase enzyme activities. Adenosylcobalamin bound to wild-type S. typhimurium LT2 apoenzyme quickly but to the mutant apoenzymes with greater difficulty. M dK affected binding between adenosylcobalamin and propanediol dehydratase apoenzyme and/or caused the mutants to produce lower amounts of apoenzyme than LT2.
Glycerol can serve as both a good substrate and a potent inactivator for propanediol dehydratase. The substrate is dehydrated to P-hydroxypropionaldehyde with concomitant inactivation of the enzyme. However, the inactivated enzyme situ was rapidly and continually reactivated by adenosine 5'-ariphosphate (ATP) and MnCb in the presence of free adenosylcobalamin. The gene or genes responsible for this reactivation system were blocked by insertion mutagenesis, and the mutations were located near 75.5 minutes on the Salmonella genetic map.