Provides the precursors necessary for DNA synthesis. Catalyzes the biosynthesis of deoxyribonucleotides from the corresponding ribonucleotides. [thioredoxin]-disulfide + a 2'-deoxyribonucleoside 5'-diphosphate + H2O = [thioredoxin]-dithiol + a ribonucleoside 5'-diphosphate Binds 2 iron ions per subunit. Genetic information processing; DNA replication. Heterodimer of a large and a small chain. Expressed in roots, rosette leaves, stems and flowers. No visible phenotype, due to the redundancy with RNR2A and TSO2. Rnr2a and rnr2b double mutants have no visible phenotype. Belongs to the ribonucleoside diphosphate reductase small chain family. In cv. Columbia, could be the product of a pseudogene (AC P0DKH2) due to a frameshift in position 140. The resulting shorter protein lacks the conserved features of the family.