Current model for Fe-S cluster assembly by the Suf system. Current model for Fe-S cluster assembly by the Suf system. (A) SufS and SufE mobilize sulfur for donation to SufB (as part of SufBCD) and SufA. (B) An Fe-S cluster can form on both SufA and SufB. The Fe-S cluster on SufA can be transferred to an apoprotein. The SufB Fe-S cluster could be transferred to an apoprotein or could participate in redox processes during cluster assembly (e−). Dotted arrows indicate possible donation of iron by SufD or by direct binding to SufA. The exact role of SufC ATPase activity is unknown. Carla Ayala-Castro et al. Microbiol. Mol. Biol. Rev. 2008; doi:10.1128/MMBR.00034-07