Effects QB disrupting the 21 kDa subunit of complex I from Neurospora crassa

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 cra ssa. Mitochondria from the nuo21 mutant lack this specific protein but reta in other subunits of complex I in approximately normal amounts. In addition , this mutant is able to assemble an almost intact enzyme. The electron tra nsfer activities from NADH to artificial accepters of mitochondrial membran es from nuo21 differ from those of the wildtype strain, suggesting that the absence of the 21 kDa polypeptide results in conformational changes in com plex I. Nevertheless, complex I of nuo21 is able to perform NADH:ubiquinone reductase activity, as judged by the observation that the respiration of m utant mitochondria is sensitive to inhibition by rotenone. We discuss these findings in relation to the involvement of complex I in mitochondrial dise ases.