Ribonucleotide reductase (EC 1.17.4.1) [1,2] 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).

The small chain binds two iron atoms [3] (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.

We have 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.