THE 28.5-KDA IRON-SULFUR PROTEIN OF MITOCHONDRIAL COMPLEX-I IS ENCODED IN THE NUCLEUS IN PLANTS

The intrinsic 28.5-kDa iron-sulfur protein of complex I in the mitocho ndrial respiratory chain is encoded in the nucleus in animals and fung i, but specified by a mitochondrial gene in trypanosomes. In plants, t he homologous protein is now found to be encoded by a single-copy nucl ear gene in Arabidopsis thaliana and by two nuclear genes in potato. T he cysteine motifs involved in binding two iron-sulfur clusters are co nserved in the plant protein sequences. The locations of the seven int rons, with sizes between 60 and 1700 nucleotides, are identical in the A. thaliana and the two potato genes, while their primary sequences d iverse 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, differenc es in T content may instead serve to discriminate exons and introns. I n potato, both genes are expressed, with the highest levels found in f lowers. Sequence similarities between the homologous nuclear and mitoc hondrial genes indicate that the nuclear forms in animals and plants o riginate from the endosymbiont genome.