Novel zinc-containing ferredoxin family in thermoacidophilic archaea

The dicluster-type ferredoxins from the thermoacidophilic archaea such as Thermoplasma acidophilum and Sulfolobus sp. are known to contain an unusually long extension of unknown function in the N-terminal region. Recent x-ray structural analysis of the Sulfolobus ferredoxin has revealed the presence of a novel zinc center, which is coordinated by three histidine ligand residues in the N-terminal region and one aspartate in the ferredoxin core domain. We report here the quantitative metal analyses together with electron paramagnetic resonance and resonance Raman spectra of T. acidophilum ferredoxin, demonstrating the presence of a novel zinc center in addition to one [3Fe-4S] and one [4Fe-4S] cluster (Fe/Zn = 6.8 mol/mol). A phylogenetic tree constructed for several archaeal monocluster and dicluster type ferredoxins suggests that the zinc-containing ferredoxins of T. acidophilum and Sulfolobus sp. form an independent subgroup, which is more distantly related to the ferredoxins from the hyperthermophiles than those from the methanogenic archaea, indicating the existence of a novel group of ferredoxins, namely, a "zinc-containing ferredoxin family" in the thermoacidophilic archaea. Inspection of the N-terminal extension regions of the archaeal zinc-containing ferredoxins suggested strict conservation of three histidine and one aspartate residues as possible ligands to the novel zinc center.