Biosynthesis of selenate reductase in Salmonella enterica: critical roles for the signal peptide and DmsD

Salmonella enterica serovar Typhimurium is a Gram negative bacterium with a flexible respiratory capability. Under anaerobic conditions, S. enterica can utilise a range of terminal electron acceptors to sustain respiratory electron transport chains, including the potentially toxic compound selenate. The S. enterica selenate reductase is a membrane-bound enzyme encoded by the ynfEFGH-dmsD operon. The active enzyme is predicted to comprise at least three subunits where YnfE is a molybdenumcontaining catalytic subunit. The YnfE protein is synthesised with an N-terminal twinarginine signal peptide and biosynthesis of the enzyme is co-ordinated by a signalpeptide-binding chaperone called DmsD. In this work, the interaction between DmsD and the YnfE signal peptide has been studied by chemical crosslinking. These experiments were complemented by genetic approaches, which identified the DmsD binding epitope within the YnfE signal peptide. YnfE signal peptide residues L24 and A28 were shown to be important for assembly of an active selenate reductase. Conversely, a random genetic screen identified the DmsD V16 residue as being important for signal peptide recognition and selenate reductase assembly.