Component of the phosphorylative electron transport system with polysulfide as the terminal acceptor. Binds 1 [4Fe-4S] cluster. Binds 1 molybdenum-bis(molybdopterin guanine dinucleotide) (Mo-bis-MGD) cofactor per subunit. Functional polysulfide reductase is made up of three different (A, B, and C) subunits. Predicted to be exported by the Tat system. The position of the signal peptide cleavage has been experimentally proven. Belongs to the prokaryotic molybdopterin-containing oxidoreductase family.