ABSTRACT
The two strands of the circular mtDNA chromosome have an asymmetric distri bution of Gs and Cs generating heavy (H)- and light (L)-strands. Gene products encoded by the L-strand include one polypeptide (MTND6) and nine tRNAs whereas gene products of the H-strand include 12 polypeptides, 2 rRNAs and 13 tRNAs. Seven of the polypeptides are subunits of complex I (NADH-dehydrogenaseubiquinone reductase) and are named MTND 1, 2, 3, 4, 4L, 5 and 6; cytochrome b (MTCYB) is part of complex III (ubiquinone-cytochrome c reductase); three polypep tides, COI (MTCOI), COU (MTC02) and COIII (MTC03), are the catalytic subunits of complex IV (cytochrome c oxidase); and finally, ATPase-6 (MTATP6) and ATPase-8 (MTATP8) are components of complex V. All the subunits of complex II (succinate dehydrogenase-ubiquinone reductase) are nuclear DNA (nDNA)-encoded (Leckschat et al., 1993; Hirawake et al., 1994; Morris et al., 1994;
Au et al., 1995). In order to produce functionally active complexes, the mtDNAencoded subunits of each complex must interact with several subunits, which are encoded by nDNA and then imported into mitochondria from the cytoplasm (Glick and Schatz, 1991).
