A bifunctional cobalamin reductase that converts cob(III)alamin to cob(II)alamin and then to cob(I)alamin in the bacterial microcompartment (BMC) dedicated to 1,2-propanediol (1,2-PD) degradation. PduS and PduO allow regeneration of the adenosylcobalamin cofactor within the BMC (PubMed:21103360). Cobalamin reduction probably occurs spontaneously in the presence of free reduced flavin nucleotides, this protein may be involved in electron transfer for this reduction (By similarity). Expression of a cosmid containing the full 21-gene pdu operon in E.coli allows E.coli to grow on 1,2-propanediol (1,2-PD) with the appearance of BMCs in its cytoplasm. The 1,2-PD-specific bacterial microcompartment (BMC) concentrates low levels of 1,2-PD catabolic enzymes, concentrates volatile reaction intermediates thus enhancing pathway flux and keeps the level of toxic, mutagenic propionaldehyde low. Binds 2 [4Fe-4S] clusters (PubMed:21103360) (By similarity). The two centers are coupled but must possess different redox potentials (PubMed:21103360). Binds one FMN non-covalently per monomer. E(0) is -262 mV for aerobically purified protein (only with FMN). E(0) is -150 mV for anaerobically purified protein (with FMN and 4Fe-4S centers). Polyol metabolism; 1,2-propanediol degradation. Monomer, forms a complex with PduO (By similarity). Interacts with PduT, probably via the N-terminus of PduS (PubMed:21103360). Belongs to the PduS cobalamin reductase family.