Mitochondrial Complex I (or NADH-ubiquinone reductase, EC 1.6. 5.3) is the first multimeric complex of the respiratory chain. It catalyzes the NADH oxidation with concomitant ubiquinone reduction and proton ejection out of the mitochondria. Mammalian mitochondrial Complex I is an assembly of at least 43 different subunits. Seven out of the 43 subunits are encoded by the mitochondrial genome; the remainder are the products of nuclear genes (Fearnley and Walker 1992). Defects in Complex I are involved in the onset of an increasing number of important cytopathologies (Wallace 1995). Tissues significantly affected are skeletal muscle and/or the nervous system. In some cases, Complex I deficiencies have been correlated with quantitative decrease or even absence of specific subunits of the complex in tissue samples (Moreadith et al. 1987; Schapira et al. 1988; Slipetz et al. 1991). However, the DNA mutations that may affect the nuclear genes for these syndromes are not known.

So far, the genomic organization has been reported for only three nuclear genes coding for human Complex I subunits: the NDUFV2, NDUFA1, and NDUFV3 genes, which code for the 24-kDa, the MWFE and the 10-kDa subunits respectively. These genes were mapped to Chromosomes (Chrs) 18p11. 3, Xq24, and 21q22. 3 respectively (De Coo et al. 1995; Hattori et al. 1995; Zhuchenko et al. 1996; De Coo et al. 1997). Chromosomal localization has been reported for seven other genes that code for the following subunits: B13, B22, 75 kDa, 51 kDa, 39 kDa, 23 kDa (or TYKY subunit), and 20 kDa (or PSST subunit). They are localized to Chrs 7q32, 8q13. 3, 2q33–34, 11q13, 12p, 11q13, and 19p13. 3 respectively (Duncan et al. 1992; Spencer et al. 1992; Baens et al. 1993; Gu et al. 1996, Hyslop et al. 1996; Procaccio et al. 1997; Russell et al. 1997). We have sequenced and characterized the cDNA for the 49-kDa subunit and assigned the chromosomal localization of the corresponding gene. We have isolated six cDNA clones coding for the 49-kDa subunit after screening of a human cDNA lymphocyte library (Clontech, Palo Alto, CA, USA) with the bovine 49-kDa cDNA as a probe (Genbank accession number X14338). After sequencing, the positive clones were shown to contain a DNA fragment corresponding to the 5 end of the cDNA coding for the 49-kDa subunit (Fig. 1: from 5 end to the internal XhoI site at position 971 bp). PCR experiments, using DNA from the cDNA library as template, were performed to amplify the cDNA 3 end [forward primer: For2 (Fig. 1) and a reverse primer hybridizing to the plasmid]. The sequences of PCR fragments from three different PCR experiments and those of the 5 end cDNA clones were merged,