We have cloned and inactivated in vivo, by repeat-induced point mutations, the nuclear gene encoding a 21 kDa subunit of complex I from Neurospora crassa. Mitochondria from the nuo21 mutant lack this specific protein but retain other subunits of complex I in approximately normal amounts. In addition, this mutant is able to assemble an almost intact enzyme. The electron transfer activities from NADH to artificial acceptors of mitochondrial membranes from nuo21 differ from those of the wild-type strain, suggesting that the absence of the 21 kDa polypeptide results in conformational changes in complex I. Nevertheless, complex I of nuo21 is able to perform NADH:ubiquinone reductase activity, as judged by the observation that the respiration of mutant mitochondria is sensitive to inhibition by rotenone. We discuss these findings in relation to the involvement of complex I in mitochondrial diseases.

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