Nitrogenase (EC 1.18.6.1) [1] is the enzyme system responsible for biological nitrogen fixation. Nitrogenase is an oligomeric complex which consists of two components: component 2 is an homodimer of an iron-sulfur protein, while component 1 which contains the active site for the reduction of nitrogen to ammonia exists in three different forms:

• A molybdenum-iron containing protein (MoFe). The MoFe protein is a hetero- tetramer consisting of two pairs of α (nifD) and β (nifK) subunits.
• A vanadium-iron containing protein (VFe). The VFe protein is a hexamer of two pairs each of α (vnfD), β (vnfK), and delta (vnfG) subunits.
• The third form of component 1 seems to only contain iron. Like the vanadium form it is a hexamer composed of α (anfD), β (anfK), and delta (anfG) subunits.

The α and β chains of the three types of component 1 are evolutionary related and they are also related to proteins nifE and nifN, which are most probably involved in the iron-molybdenum cofactor biosynthesis [2].

We selected as signature patterns for this family of proteins two stretches of residues which are located in the N-terminal section and which each contain a conserved cysteine thought to be one of the ligands for the metal-sulfur clusters.