Component of the PhsABC thiosulfate reductase that catalyzes the reduction of thiosulfate to sulfite and hydrogen sulfide, with menaquinol as the sole electron donor. Proton motive force (PMF) is required to drive transmembrane electron transfer within the reductase. The PhsA subunit contains the active site molybdenum-bis(molybdopterin guanine dinucleotide) (Mo-bis-MGD) cofactor. a quinone + 2 H(+) + hydrogen sulfide + sulfite = a quinol + thiosulfate Binds 1 [4Fe-4S] cluster. Binds 1 molybdenum-bis(molybdopterin guanine dinucleotide) (Mo-bis-MGD) cofactor per subunit. Composed of three subunits: PhsA, PhsB and PhsC. Predicted to be exported by the Tat system. The position of the signal peptide cleavage has not been experimentally proven. Belongs to the prokaryotic molybdopterin-containing oxidoreductase family.