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. Inhibited by phenol, paracetamol, 2,4,6-trimethylphenol, resveratrol, furfuryl mercaptan, 2-thiophenthiol, phenylhydrazine, and hydroxyurea. Genetic information processing; DNA replication. Homodimer and heterodimer with TSO2. Heterotetramer of two R1 and two R2 chains. A radical transfer pathway may occur between Tyr-125 of protein R2 and R1. Homodimer contains a dinuclear non-heme iron center and a stable tyrosyl radical essential for activity. A transfer pathway may occur between Tyr-125 of protein R2 and R1. Interacts with CSN7. Expressed in rosette leaves, cauline leaves, stems and flowers. Accumulated during S phase of the cell cycle. No visible phenotype, due to the redundancy with RNR2B and TSO2. Rnr2a and rnr2b double mutants have no visible phenotype. 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.