Respiratory-chain NADH dehydrogenase (EC 1.6.5.3) [1,2] (also known as complex I or NADH-ubiquinone oxidoreductase) is an oligomeric enzymatic complex located in the inner mitochondrial membrane which also seems to exist in the chloroplast and in cyanobacteria (as a NADH-plastoquinone oxidoreductase). Among the 25 to 30 polypeptide subunits of this bioenergetic enzyme complex there is one with a molecular weight of 20 Kd (in mammals) [3], which is a component of the iron-sulfur (IP) fragment of the enzyme. It seems to bind a 4Fe-4S iron-sulfur cluster. The 20 Kd subunit has been found to be:

• Nuclear encoded, as a precursor form with a transit peptide in mammals, and in Neurospora crassa.
• Mitochondrial encoded in Paramecium (gene psbG).
• Chloroplast encoded in various higher plants (gene ndhK or psbG).

The 20 Kd subunit is highly similar to [4]:

• Synechocystis strain PCC 6803 proteins psbG1 and psbG2.
• Subunit B of Escherichia coli NADH-ubiquinone oxidoreductase (gene nuoB).
• Subunit NQO6 of Paracoccus denitrificans NADH-ubiquinone oxidoreductase.
• Subunit 7 of Escherichia coli formate hydrogenlyase (gene hycG).
• Subunit I of Escherichia coli hydrogenase-4 (gene hyfI).

As as signature pattern we selected a highly conserved region, located in the central section of this subunit and which contains a conserved cysteine that is probably involved in the binding of the 4Fe-4S center.