DISRUPTION OF THE GENE ENCODING THE NADH-BINDING SUBUNIT OF NADH-UBIQUINONE OXIDOREDUCTASE IN NEUROSPORA-CRASSA FORMATION OF A PARTIALLY ASSEMBLED ENZYME WITHOUT FMN AND THE IRON-SULFUR CLUSTER N-3

In this study, the gene of the 51-kDa NADH-binding subunit of the mito chondrial NADH:ubiquinone oxidoreductase (complex I) in Neurospora cra ssa was inactivated by homologous replacement with a defective gene co py. The resulting mutant, nuo51, lacks the 51-kDa subunit and shows no complex I activity but still grows at one third of the wild-type grow th rate. The enzyme activity of the alternative NADH:ubiquinone oxidor eductase(s) is increased twofold while the activities of the other mit ochondrial respiratory enzymes are normal. Complex I is almost complet ely assembled except for the NADH-binding subunit and still possesses three out of the four EPR-detectable iron-sulphur clusters. Since the deleted subunit contains the sequence motif for one tetranuclear iron- sulphur cluster, the missing cluster N-3 is considered to be bound to this subunit.