Illustration of suf and isc operons, with corresponding promoters (Pisc and Psuf), which play a central role in regulation of SUF and ISC at the genetic level in bacteria. Operons depicted are from the E. coli model system. Steps are colored based on the encoded protein’s function as follows: red (regulatory), yellow (sulfur delivery), green (primary scaffold), blue (downstream Fe-S cluster delivery), cyan (electron transfer), and gray (unknown).