Multiple Roles for the Twin Arginine Leader Sequence of Dimethyl Sulfoxide Reductase of Escherichia coli*

Dimethyl sulfoxide (Me₂SO) reductase of Escherichia coli is a terminal electron transport chain enzyme that is expressed under anaerobic growth conditions and is required for anaerobic growth with Me₂SO as the terminal electron acceptor. The trimeric enzyme is composed of a membrane extrinsic catalytic dimer (DmsAB) and a membrane intrinsic anchor (DmsC). The amino terminus of DmsA has a leader sequence with a twin arginine motif that targets DmsAB to the membrane via a novel Sec-independent mechanism termed MTT for membrane targeting and translocation. We demonstrate that the Met-1 present upstream of the twin arginine motif serves as the correct translational start site. The leader is essential for the expression of DmsA, stability of the DmsAB dimer, and membrane targeting of the reductase holoenzyme. Mutation of arginine 17 to aspartate abolished membrane targeting. The reductase was labile in the leader sequence mutants. These mutants failed to support growth on glycerol-Me₂SO minimal medium. Replacing the DmsA leader with the TorA leader of trimethylamineN-oxide reductase produced a membrane-bound DmsABC with greatly reduced enzyme activity and inefficient anaerobic respiration indicating that the twin arginine leaders may play specific roles in the assembly of redox enzymes.