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Genetic evidence has indicated that Isc proteins play an important role in iron-sulfur cluster biogenesis. In particular, IscU is believed to serve as a scaffold for the assembly of a nascent iron-sulfur cluster that is subsequently delivered to target iron-sulfur apoproteins. We report the characterization of an IscU fromThermatoga maritima, an evolutionarily ancient hyperthermophilic bacterium. The stabilizing influence of a D40A substitution allowed characterization of the holoprotein. Mössbauer (δ = 0.29 ± 0.03 mm/s, ΔE_Q = 0.58 ± 0.03 mm/s), UV-visible absorption, and circular dichroism studies of the D40A protein show that T. maritima IscU coordinates a [2Fe-2S]²⁺ cluster. Thermal denaturation experiments demonstrate that T. maritima IscU is a thermally stable protein with a thermally unstable cluster. This is also the first IscU type domain that is demonstrated to possess a high degree of secondary and tertiary …