Abstract
Iron-sulfur clusters may have been the earliest catalytic cofactors on earth, and most modern organisms use them extensively. Although members of the
Archaea
produce numerous iron-sulfur proteins, the major cluster assembly proteins found in the
Bacteria
and
Eukarya
are not universally conserved in archaea. Free-living archaea do have homologs of the bacterial
apbC
and eukaryotic
NBP35
genes that encode iron-sulfur cluster carrier proteins. This study exploits the genetic system of
Salmonella enterica
to examine the in vivo functionality of
apbC
/
NBP35
homologs from three archaea:
Methanococcus maripaludis
,
Methanocaldococcus jannaschii
, and
Sulfolobus solfataricus.
All three archaeal homologs could correct the tricarballylate growth defect of an
S. enterica apbC
mutant. Additional genetic studies showed that the conserved Walker box serine and the Cys-X-X-Cys motif of the
M. maripaludis
MMP0704 protein were both required for function in vivo but that the amino-terminal ferredoxin domain was not. MMP0704 protein and an MMP0704 variant protein missing the N-terminal ferredoxin domain were purified, and the Fe-S clusters were chemically reconstituted. Both proteins bound equimolar concentrations of Fe and S and had UV-visible spectra similar to those of known [4Fe-4S] cluster-containing proteins. This family of dimeric iron-sulfur carrier proteins evolved before the archaeal and eukaryal lineages diverged, representing an ancient mode of cluster assembly.