Ribonucleotide reductase catalyzes the reductive synthesis of deoxyribonucleotides from their corresponding ribonucleotides. It provides the precursors necessary for DNA synthesis. Ribonucleotide reductase is an oligomeric enzyme composed of a large subunit (700 to 1000 residues) and a small subunit (300 to 400 residues).

Large subunit: There are regions of similarities in the sequence of the large chain from prokaryotes, eukaryotes and viruses. We have selected one of these regions as a signature pattern.Reference

The small chain binds two iron atoms (three Glu, one Asp, and two His are involved in metal binding) and contains an active site tyrosine radical. The regions of the sequence that contain the metal-binding residues and the active site tyrosine are conserved in ribonucleotide reductase small chain from prokaryotes, eukaryotes and viruses. It has been selected one of these regions as a signature pattern. It contains the active site residue as well as a glutamate and a histidine involved in the binding of iron. Reference