nrdB

nrdB is the gene that encodes the beta subunit (R2) of class I ribonucleotide reductase (RNR), an essential enzyme complex in many aerobic bacteria and some archaea. The ribonucleotide reductase enzyme converts ribonucleotides to deoxyribonucleotides, providing the precursors necessary for DNA synthesis and repair. In class I RNR, the functional enzyme consists of a large subunit (R1, encoded by nrdA) and a small subunit (R2, encoded by nrdB). The R2 subunit forms a dimer that harbors a di-iron center and a stable tyrosyl radical, which together initiate the radical-based catalysis needed to reduce ribonucleotides within the R1 subunit.

The R2 subunit contains conserved metal-binding motifs that coordinate two iron ions and support generation of the tyrosyl radical. This radical is then transferred to cysteine residues in the R1 subunit, enabling the reduction of ribonucleotides to deoxyribonucleotides. The activity and assembly of the RNR holoenzyme depend on the proper maturation of the metal center and the radical, and are influenced by cellular iron availability and oxidative conditions.

Genomic organization commonly places nrdB adjacent to nrdA, and in many bacteria the two genes are co-regulated. Some organisms possess alternative subunits (such as NrdF) under different conditions or in different RNR classes (for example, class Ib). Under anaerobic conditions or iron limitation, cells may rely on other classes of RNR (class II or III), reducing reliance on nrdB.

Overall, nrdB plays a crucial role in DNA synthesis by enabling the formation of deoxyribonucleotides through the class I ribonucleotide reductase system.