The intrinsic 28.5-kDa iron-sulfur protein of complex I in the mitochondrial respiratory chain is encoded in the nucleus in animals and fungi, but specified by a mitochondrial gene in trypanosomes. In plants, the homologous protein is now found to be encoded by a single-copy nuclear gene in Arabidopsis thaliana and by two nuclear genes in potato. The cysteine motifs involved in binding two iron-sulfur clusters are conserved in the plant protein sequences. The locations of the seven introns, with sizes between 60 and 1700 nucleotides, are identical in the A. thaliana and the two potato genes, while their primary sequences diverge considerably. The A + T contents of the intron sequences range between 61% and 73%, as is characteristic for dicot plants, but are in some instances not higher than in the adjacent exons. Here, differences in T content may instead serve to discriminate exons and introns. In potato, both genes are expressed, with the highest levels found in flowers. Sequence similarities between the homologous nuclear and mitochondrial genes indicate that the nuclear forms in animals and plants originate from the endosymbiont genome.

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The multi‐subunit enzyme complex I of the mitochondrial respiratory chain is an assembly of nuclear and organ‐ellar encoded proteins with distinct roles and functions. The nuclear encoded 28.5 kDa iron‐sulfur protein is located at the centre of electron transfer to ubiquinone. Functional importance and regulatory tolerance of this subunit were investigated in transgenic tobacco plants carrying antisense constructs driven by the CaMV 35S promoter. In all of the regenerated transgenics vegetative growth is undisturbed, while in many transformants flower development is abnormal and pollen fertility is reduced. Maximal observed suppression of the steady‐state 28.5 kDa mRNA level reaches only about 30%. Apparently, further reduction is lethal to the vegetative tobacco plants, suggesting that the 28.5 kDa subunit is regulated from the steady‐state level onwards with little tolerance and no additional possibilities for compensation. This contrasts with the higher flexibility of the NADH‐binding subunit of complex I, which vegetatively survives a 70% reduction of its mRNA level.