| Research Abstract |
IscA homologues are involved in iron-sulfur cluster biosynthesis. In the non-nitrogen-fixing cyanobacterium Synechocystis PCC6803, there are two IscA homologues, SLR1417 and SLR1565 (designated IscA1 and IscA2), of which only IscA2 exists as a protein complex with the HEAT-repeat-containing protein, SLR1O98 (IaiH). We observed that the absorption spectrum of the recombinant IscA2/IaiH complex resembles that of IscA2 alone, although it is sharper: In the presence of dithiothreitol, the [2Fe-2S] cluster of IscA2 alone, but not of the IscA2/IaiH complex, became reductively labile upon the addition of sodium ditbionite. This implies that the IscA2 moiety of the [2Fe-2S] cluster is stabilized by the presence of IaiH. The [2Fe-2S] cluster of the IscA2/IaiH complex was destabilized by sodium dithionite in the absence of dithiothreitol, suggesting that the in vivo stability of the iron-sulfur cluster in the IscA2/IaiH complex is influenced by the redox state of cellular thiols. When any one of
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three conserved cysteine residues in IscA2, potential ligands for the [2Fe-2S] cluster, was replaced with serine, the amount of assembled [2Fe2S] cluster and protein complex was significantly reduced inE co/i cells. The cysteine mutated IscA2/IaiH complexes that were present all contained a [2Fe-2S]-like cluster suggesting that the assembly of a stable iron-sulfur cluster bound to IscA2 is required for efficient and stable complex formation.
We also identified two chloroplast-localized NifU-like proteins, AtCnfU-V and AtCnfU-IVb, from Arabidopsis thaliana with high sequence similarity to a cyanobaderial NifU-like protein that was proposed to serve as a molecular scaffold. AtCnfU-V is constitutively expressed in several tissues of Arabidopsis, whereas the expression of AtCnfU-IVb is prominent in the aerial parts. Mutant Arabidopsis lacking AtCnfU-V exhllited a dwarf phenotype with faint pale-giten leaves and had drastically impaired photosystem I accumulation. Chloroplasts in the mutants also showed a decrease in both the amount of ferredoxin, a major electron carrier of the stroma that contains a [2Fe-2S] cluster, and in the in vitro activity of iron-sulfur cluster insertion into apo-fenedoxin. When expressed in E. ccli cells, AtG~f U-V formed a homodimer canying a [2Fe-2S]-like cluster, and this cluster could be transferred to apo-ferredoxin in vitro to form holo-ferredoxin. We propose that AtCnfU has an important function as a molecular scaffold for iron-sulfur cluster biosynthesis in chlotoplasts and thereby is required for biogenesis of ferredoxin and photosystem I. Less
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