Catalyzes the reduction of ribonucleotides to deoxyribonucleotides. May function to provide a pool of deoxyribonucleotide precursors for DNA repair during oxygen limitation and/or for immediate growth after restoration of oxygen. [thioredoxin]-disulfide + a 2'-deoxyribonucleoside 5'-diphosphate + H2O = [thioredoxin]-dithiol + a ribonucleoside 5'-diphosphate 5'-deoxyadenosylcobalamine (coenzyme B12). Homotetramer. Expressed during the exponential phase of growth. Belongs to the ribonucleoside diphosphate reductase class-2 family.