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 subunit. Localization to the cytoplasm is CSN- and checkpoint-dependent and is required for an efficient DNA damage response. Belongs to the ribonucleoside diphosphate reductase small chain family.