The last step in coenzyme B12 synthesis is localized to the cell membrane in bacteria and archaea

Abstract

InSalmonella enterica, the last step of the synthesis of adenosylcobamide is catalysed by the cobalamin synthase enzyme encoded by thecobSgene of this bacterium. Overexpression of theS. enterica cobSgene inEscherichia colielicited the accumulation of the phage shock protein PspA, a protein whose expression has been linked to membrane stress. Resolution of inner and outer membranes ofS. entericaby isopycnic density ultracentrifugation showed CobS activity associated with the inner membrane, a result that was confirmed using antibodies against CobS. Computer analysis of the predicted amino acid sequence of CobS suggested it was an integral membrane protein. Results of experiments performed with strains carrying plasmids encoding CobS–alkaline phosphatase or CobS–β-galactosidase protein fusions were consistent with the membrane localization of the CobS protein. Modifications to the predicted model were made based on data obtained from experiments using protein fusions. The function encoded by thecobSorthologue in the methanogenic archaeonMethanobacterium thermoautotrophicumstrain ΔH compensated for the lack of CobS during cobalamin synthesis incobSstrains ofS. enterica. Cobalamin synthase activity was also detected in a membrane preparation ofM. thermoautotrophicum. It was concluded that the assembly of the nucleotide loop of adenosylcobamides in archaea and bacteria is a membrane-associated process. Possible reasons for the association of adenosylcobamide biosynthetic enzymes with the cell membrane are discussed.