Provides the precursors necessary for DNA synthesis. Catalyzes the biosynthesis of deoxyribonucleotides from the corresponding ribonucleotides. R2 contains the tyrosyl radical required for catalysis. [thioredoxin]-disulfide + a 2'-deoxyribonucleoside 5'-diphosphate + H2O = [thioredoxin]-dithiol + a ribonucleoside 5'-diphosphate Binds 2 iron ions per subunit. Inhibited by hydroxyurea, leads to dNTP depletion, replication fork arrest and genomic instability. Genetic information processing; DNA replication. Tetramer of two alpha (R1) and two beta (R2) subunits. The B1 protein is a dimer of alpha subunits. A radical transfer pathway occurs between Tyr-123 of R2 and R1. Induced 4.2-fold by hydroxyurea (at protein level). E.coli produces two separate class I enzymes. This one is the functional enzyme during growth. A substrate-binding catalytic site, located on R1, is formed only in the presence of the second subunit R2. Belongs to the ribonucleoside diphosphate reductase small chain family.