Characterization of the 25-kilodalton subunit of the energy-transducing NADH-ubiquinone oxidoreductase of Paracoccus denitrificans: sequence similarity to the 24-kilodalton subunit of the flavoprotein fraction of mammalian complex I.

The NADH dehydrogenase complex isolated from Paracoccus denitrificans is composed of approximately 10 unlike polypeptides [Yagi, T. (1986) Arch. Biochem. Biophys. 250, 302-311]. Structural genes encoding the subunits of this enzyme complex constitute at least one gene cluster [Xu, X., Matsuno-Yagi, A., & Yagi, T. (1991) Biochemistry 30, 6422-6428]. The 25-kDa subunit (NQO2), which has been isolated from sodium dodecyl sulfate-polyacrylamide gels, is a polypeptide of this enzyme complex. The partial N-terminal amino acid sequence and amino acid composition of the NQO2 subunit have been determined. On the basis of the amino acid sequence, the NQO2 gene was found to be located 1.7 kilobase pairs upstream of the gene for NADH-binding subunit (NQO1). The complete nucleotide sequence of the NQO2 gene was determined. It is composed of 717 base pairs and codes for 239 amino acid residues with a calculated molecular weight of 26,122. The NQO2 subunit is homologous to the Mr 24,000 subunit of the mammalian mitochondrial NADH-ubiquinone oxidoreductase which bears an electron paramagnetic resonance-visible binuclear iron-sulfur cluster (probably cluster N1b). Comparison of the predicted amino acid sequence of the Paracoccus NQO2 subunit with those of its mammalian counterparts suggests putative binding sites for the iron-sulfur cluster. In addition, nucleotide sequencing shows the presence of two unidentified reading frames between the NQO1 and NQO2 genes. These are designated URF1 and URF2 and are composed of 261 and 642 base pairs, respectively. The possible function of the protein coded for the URF2 is discussed.