Provides the precursors necessary for DNA synthesis. Catalyzes the biosynthesis of deoxyribonucleotides from the corresponding ribonucleotides. R1 contains the binding sites for both substrates and allosteric effectors and carries out the actual reduction of the ribonucleotide. Ribonucleotide reductase (RNR) complex function is essential for efficient organellar DNA degradation in pollen. Involved in chloroplast division. [thioredoxin]-disulfide + a 2'-deoxyribonucleoside 5'-diphosphate + H2O = [thioredoxin]-dithiol + a ribonucleoside 5'-diphosphate Under complex allosteric control mediated by deoxynucleoside triphosphates and ATP binding to separate specificity and activation sites on the large subunit. The type of nucleotide bound at the specificity site determines substrate preference. It seems probable that ATP makes the enzyme reduce CDP and UDP, dGTP favors ADP reduction and dTTP favors GDP reduction. Stimulated by ATP and inhibited by dATP binding to the activity site (By similarity). Genetic information processing; DNA replication. Heterotetramer of two large/R1 and two small/R2 subunits. A radical transfer pathway may occur between 'Tyr-125' of protein R2 and R1. Highly expressed in actively growing tissues such as young leaves, shoot apices, inflorescences and carpels. Very low expression in cotyledons, adult and cauline leaves and senescent leaves. Basal expression regulated by ATR/RAD3. Can be induced by another pathway when dNTPs levels are low. Contains a disulfide bonds (Probable). Binding of the substrate occurs primarily when the active-site cysteines are reduced. Lethal when homozygous. RNAi-mediated knockdown causes severe developmental defects in seedlings failing to develop beyond the four-leaf stage. Two distinct regulatory sites have been defined: one controls substrate specificity and the other regulates the overall catalytic activity. 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 large chain family.