Iron-sulphur cluster assembly in plants: distinct NFU proteins in mitochondria and plastids from Arabidopsis thaliana

Recent results are in favour of a role for NFU-like proteins in Fe–S cluster biogenesis. These polypeptides share a conserved CXXC motif in their NFU domain. In the present study, we have characterized Arabidopsis thalianaNFU1–5 genes. AtNFU proteins are separated into two classes. NFU4 and NFU5 are part of the mitochondrial type, presenting a structural organization similar to Saccharomyces cerevisiae Nfu1p. These proteins complement a Δisu1Δnfu1 yeast mutant and NFU4 mitochondrial localization was confirmed by green fluorescent protein fusion analysis. AtNFU1–3 represent a new class of NFU proteins, unique to plants. These polypeptides are made of two NFU domains, the second having lost its CXXC motif. AtNFU1–3 proteins are more related to Synechocystis PCC6803 NFU-like proteins and are localized to plastids when fused with the green fluorescent protein. NFU2 and/or NFU3 were detected in leaf chloroplasts by immunoblotting. NFU1 and NFU2 are functional NFU capable of restoring the growth of a Δisu1Δnfu1 yeast mutant, when addressed to yeast mitochondria. Furthermore, NFU2 recombinant protein is capable of binding a labile 2Fe–2S cluster in vitro. These results demonstrate the presence of distinct NFU proteins in Arabidopsis mitochondria and plastids. Such results suggest the existence of two different Fe–S assembly machineries in plant cells.