The mitochondrial DNA of the Nicotiana sylvestris CMSII mutant carries a 72-kb deletion comprising the single copy nad7 gene that encodes the NAD7 subunit of the respiratory complex I (NADH-ubiquinone oxidoreductase). CMSII plants lack rotenone-sensitive complex I activity and are impaired in physiological and phenotypical traits. To check whether these changes directly result from the deletion of nad7, we constructed CMS transgenic plants (termed as CMSnad7) carrying an edited nad7 cDNA fused to the CAMV 35 S promoter and to a mitochondrial targeting sequence. The nad7 sequence was transcribed and translated and the NAD7 protein directed to mitochondria in CMSnad7 transgenic plants, which recovered both wild type morphology and growth features. Blue-native/SDS gel electrophoresis and enzymatic assays showed that, whereas fully assembled complex I was absent from CMSII mitochondria, a functional complex was present in CMSnad7 mitochondria. Furthermore, a supercomplex involving complex I and complex III was present in CMSnad7 as in the wild type. Taken together, these data demonstrate that lack of complex I in CMSII was indeed the direct consequence of the absence of nad7. Hence, NAD7 is a key element for complex assembly in plants. These results also show that allotopic expression from the nucleus can fully complement the lack of a mitochondrialencoded complex I gene.

Cells have developed defense mechanisms against heavy metals, which are highly toxic compounds. In animals and fungi small Cys-rich proteins called metallothioneins are induced by heavy metals (Thiele, 1992). These proteins can coordinate heavy metals by thiol bonds and confer heavymetal tolerance on these organisms. Similar proteins and genes have been identified in plants but their function remains unclear (Zenk, 1996; Foley et al., 1997). Moreover, plants contain another class of metal-binding ligands called phytochelatins. Phytochelatins are poly(␥-glutamylcysteinyl)glycines synthesized from GSH and are clearly involved in detoxification processes (for review, see Zenk, 1996). A recent study has shown that TRX-like genes can confer heavy-metal tolerance on Escherichia coli-sensitive