A protein that aids in conversion of 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 (PubMed:15817784, PubMed:20656910). The latter step requires PduO. No free cob(I)alamin is released, suggesting a complex is formed with PduO that finishes conversion to adenosylcobalamin. PduS and PduO allow regeneration of the adenosylcobalamin cofactor within the BMC (PubMed:15817784). Another study showed reduction of cob(II)alamin to cob(I)alamin in the absence of PduO. Both reactions require NADH. Cyanocobalamin (CN-Cbl) is not a substrate for the first reaction (PubMed:20656910). Cobalamin reduction probably occurs spontaneously in the presence of free reduced flavin nucleotides, this protein may be involved in electron transfer for this reduction (Probable). 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 (By similarity). The two centers are coupled but must possess different redox potentials (By similarity). Binds one FMN non-covalently per monomer. Optimum pH for cob(III)alamin reductase activity is pH 9.5, no major effect of pH was seen on cob(II)alamin reductase activity between pH 7-10. Optimum temperature is 37 degrees Celsius for cob(III)alamin reductase and 42 degrees Celsius for cob(II)alamin reductase. Polyol metabolism; 1,2-propanediol degradation. Monomeric when purified anaerobically, dimeric under aerobic conditions (PubMed:20656910). Forms a complex with PduO (PubMed:15817784, PubMed:20656910). Interacts with PduT, probably via the N-terminus of PduS (By similarity). BMC production is induced by growth on 1,2-PD vitamin B12 medium. Grows more slowly and to a lower cell density than wild-type. Bacterial microcompartments (BMC) 100-200 nm in cross section are formed during aerobic growth on minimal 1,2-PD-B12 or anaerobic growth on 1,2-PD-tetrathionate medium, but not during aerobic growth on glucose, anerobic growth on glucose or pyruvate-tetrathionate (PubMed:10498708). BMCs can constitute up to 10% of total cell protein (PubMed:12923081). Belongs to the PduS cobalamin reductase family.