Three different genes encode the iron-sulfur subunit of succinate dehydrogenase in Arabidopsis thaliana

The iron-sulfur protein is an essential component of mitochondrial complex II (succinate dehydrogenase, SDH), which is a functional enzyme of both the citric acid cycle and the respiratory electron transport chain. This prote in is encoded by a single-copy nuclear gene in mammals and fungi and by a m itochondrial gene in Rhodophyta and the protist Reclinomonas americana. In Arabidopsis thaliana, the homologous protein is now found to be encoded by three nuclear genes. Two genes (sdh2-1 and sdh2-2) likely arose from a rela tively recent duplication event since they have similar structures, encode nearly identical proteins and show similar expression patterns. Both genes are interrupted by a single intron located at a conserved position. Express ion was detected in all tissues analysed, with the highest steady-state mRN A levels found in flowers and inflorescences. In contrast, the third gene ( sdh2-3) is interrupted by 4 introns, is expressed at a low level, and encod es a SDH2-3 protein which is only 67% similar to SDH2-1 and SDH2-2 and has a different N-terminal presequence. Interestingly, the proteins encoded by these three genes are probably functional because they are highly conserved compared with their homologues in other organisms. These proteins contain the cysteine motifs involved in binding the three iron-sulfur clusters esse ntial for electron transport. Furthermore, the three polypeptides are found to be imported into isolated plant mitochondria.