Part of the SUF-like system that mediates the biosynthesis of iron-sulfur (Fe-S) clusters. Acts as a sulfurtransferase and thus transfers sulfur from SufS to SufB (PubMed:29292548). Mechanistically, the transfer from SufS to SufU is triggered by zinc-ligand swapping that provides a free thiol from SufU to accept sulfur from SufS (PubMed:29235855). Bind 1 Zn(2+) per monomer. Interacts with SufS; this interaction enhances SufS cysteine desulfurase activity. Interacts with frataxin/Fra (PubMed:27382962). Essential, it cannot be deleted. Upon depletion cells grow very slowly while aconitase and succinate dehydrogenase, both of which contain Fe-S clusters, have decreased activity. Upon depletion no change in reactive oxygen species is observed, while a modified bacillibactin (BB), an endogenous siderophore, is produced. Belongs to the NifU family. Was originally thought to be an iron-sulfur cluster assembly scaffold protein (PubMed:20097860, PubMed:21236255). It is now thought to be a zinc-dependent sulfur transfer protein (PubMed:24321018). The inhibition of SufS activity by the Cys-41 mutation in this protein has been described as competitive and non-competitive inhibition (PubMed:21236255, PubMed:24321018).