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The nad4 Gene of Maize Mitochondria Is Highly Conserved

Complex I (NADHubiquinone oxidoreductase) of the mitochondrial electron transport chain in higher plants is com-posed of at least 26 different subunits (Leterme and Boutry, 1993), most of which are encoded by the nucleus. Severa1 subunits, however, are coded for by the mitochondrial DNA. We report here the cloning and sequencing of the maize (Zea mays) subunit 4 gene of this complex (Table I). The maize mitochondrial nad4 locus was found to be present on an 8.0-kb XhoI restriction fragment and an adjacent 4.2-kb XhoI restiction fragment in the cms-T genotype. The 8.0-kb XhoI restriction fragment was cloned from maize cms-T mitochondrial DNA. To obtain the complete nad4 locus, a 3’ overlapping 5.5-kb KpnIIHindIII fragment was subcloned from the cosmid IIIDIT (Fauron et al., 1989). Sequence analyses indicated that the nad4 gene of maize has four exons and three introns. The XhoI site separating the 8- and 4.2-kb XhoI fragments is located within the third intron. Exons 1, 2, and 3 of the maize nad4 gene are located only on the 8-kb XhoI fragment, but sequences homologous to exon 4 are duplicated within the cms-T mitochondrial genome of maize. An exon 4-specific hybridization probe not only detects the 4.2-kb XhoI fragment adjacent to the 8.0-kb XhoI fragment but also a 3.2-kb XhoI restriction fragment (not shown). The intron/exon borders were detennined by two methods: known (Lamattina