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 49 Kd (in mammals), which is the third largest subunit of complex I and is a component of the iron-sulfur (IP) fragment of the enzyme. It seems to bind a 4Fe-4S iron-sulfur cluster. The 49 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 protozoan such as Paramecium (ORF 400), Leishmania and Trypanosoma (MURF 3).
• Chloroplast encoded in various higher plants (ORF 392).

The 49 Kd subunit is highly similar to [3,4]:

• Subunit D of Escherichia coli NADH-ubiquinone oxidoreductase (gene nuoD).
• Subunit NQO4 of Paracoccus denitrificans NADH-ubiquinone oxidoreductase.
• Subunit 5 of Escherichia coli formate hydrogenlyase (gene hycE).
• Subunit G of Escherichia coli hydrogenase-4 (gene hyfG).

As as signature pattern we selected a highly conserved region, located in the N-terminal section of this subunit.