The minimal SUF system can substitute for the canonical iron-sulfur cluster biosynthesis systems by using inorganic sulfide as the sulfur source.

Biosynthesis of iron-sulfur (Fe-S) clusters is indispensable for living cells. Three biosynthesis systems termed NIF, ISC and SUF have been extensively characterized in both bacteria and eukarya. For these L-cysteine is the sulfur source. A bioinformatic survey suggested the presence of a minimal SUF system composed of only two components, SufB* (a putative ancestral form of SufB and SufD) and SufC, in anaerobic archaea and bacteria. Here, we report the successful complementation of an Escherichia coli mutant devoid of the usual ISC and SUF systems upon expression of the archaeal sufB*C genes. Strikingly, this heterologous complementation occurred under anaerobic conditions only when sulfide was supplemented to the culture media. Mutational analysis and structural predictions suggest that the archaeal SufB*C most likely forms a SufB*2C2 complex and serves as the scaffold for de novo Fe-S cluster assembly using the essential Cys and Glu residues conserved between SufB* and SufB, in conjunction with a His residue shared between SufB* and SufD. We also demonstrate artificial conversion of the SufB*2C2 structure to the SufBC2D type by introducing several mutations to the two copies of sufB*. Our study thus elucidates the molecular function of this minimal SUF system and suggests that it is the evolutionary prototype of the canonical SUF system.