Provides the precursors necessary for DNA synthesis. Catalyzes the biosynthesis of deoxyribonucleotides from the corresponding ribonucleotides (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. Heterodimer of a large and a small subunit. Expression is detected in growing cells and decreases dramatically at the onset of development. The rnrB transcript reappears after the cells have formed multicellular aggregates. Belongs to the ribonucleoside diphosphate reductase small chain family. The gene for this protein is duplicated in strains AX3 and AX4. These strains contain a duplication of a segment of 750 kb of chromosome 2 compared to the corresponding sequence in strain AX2.