The Campylobacter jejuni NADH:ubiquinone oxidoreductase (complex I) utilizes flavodoxin rather than NADH

Campylobacter jejuni encodes 12 of the 14 subunits that make up the respiratory enzyme NADH:ubiquinone oxidoreductase (also called complex I). The two nuo genes not present in C. jejuni encode the NADH dehydrogenase, and in their place in the operon are the novel genes designated Cj1575c and Cj1574c. A series of mutants was generated in which each of the 12 nuo genes (homologues to known complex I subunits) was disrupted or deleted. Each of the nuo mutants will not grow in amino acid-based medium unless supplemented with an alternative respiratory substrate such as formate. Unlike the nuo genes, Cj1574c is an essential gene and could not be disrupted unless an intact copy of the gene was provided at an unrelated site on the chromosome. A nuo deletion mutant can efficiently respire formate but is deficient in alpha-ketoglutarate respiratory activity compared to the wild type. In C. jejuni, alpha-ketoglutarate respiration is mediated by the enzyme 2-oxoglutarate:acceptor oxidoreductase; mutagenesis of this enzyme abolishes alpha-ketoglutarate-dependent O2 uptake and fails to reduce the electron transport chain. The electron acceptor for 2-oxoglutarate:acceptor oxidoreductase was determined to be flavodoxin, which was also determined to be an essential protein in C. jejuni. A model is presented in which CJ1574 mediates electron flow into the respiratory transport chain from reduced flavodoxin and through complex I.