SufA from Erwinia chrysanthemi. Characterization of a scaffold protein required for iron-sulfur cluster assembly

SufA is a component of the recently discovered suf operon, which has been shown to play an important function in bacteria during iron-sulfur cluster biosynthesis and resistance to oxidative stress. The SufA protein from Erwinia chrysanthemi, a Gram-negative plant pathogen, has been purified to homogeneity and characterized. It is a homodimer with the ability to assemble rather labile [2Fe-2S] and [4Fe-4S] clusters as shown by Mössbauer spectroscopy. These clusters can be transferred to apoproteins such as ferredoxin or biotin synthase during a reaction that is not inhibited by bathophenanthroline, an iron chelator. Cluster assembly in these proteins is much more efficient when iron and sulfur are provided by holoSufA than by free iron sulfate and sodium sulfide. We propose the function of SufA is that of a scaffold protein for [Fe-S] cluster assembly and compare it to IscA, a member of the isc operon also involved in cluster biosynthesis in both prokaryotes and eukaryotes. Mechanistic and physiological implications of these results are also discussed.