Plays a pivotal role in cell survival by repairing damaged DNA in a p53/TP53-dependent manner. Supplies deoxyribonucleotides for DNA repair in cells arrested at G1 or G2. Contains an iron-tyrosyl free radical center required for catalysis. Forms an active ribonucleotide reductase (RNR) complex with RRM1 which is expressed both in resting and proliferating cells in response to DNA damage (By similarity). [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. Heterotetramer with large (RRM1) subunit. Interacts with p53/TP53. Interacts with RRM1 in response to DNA damage (By similarity). Translocates from cytoplasm to nucleus in response to DNA damage. Belongs to the ribonucleoside diphosphate reductase small chain family.