Provides the precursors necessary for DNA synthesis. Catalyzes the biosynthesis of deoxyribonucleotides from the corresponding ribonucleotides. Involved in DNA damage repair and programmed cell death inhibition. [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. Homodimer and heterodimer with RNR2A. Heterotetramer of two R1 and two R2 chains (By similarity). Interacts with CSN7 (via C-terminal tail). Cytoplasmic in adult cells, and nuclear in meristematic regions and in all tissues following DNA damage. The nuclear exclusion is dependent on CSN7. Expressed in roots, cauline and rosette leaves, stems and flowers. Expressed predominantly at the S-phase of the cell cycle. Embryo lethality when homozygous. 'TSO' means 'ugly' in Chinese. Belongs to the ribonucleoside diphosphate reductase small chain family.