The assembly of iron-sulfur (Fe-S) clusters, a key step in the post-translational matu-ration of Fe-S proteins, is mediated by a complex apparatus. In E. coli, this process involves two independent systems called ISC and SUP encoded by the iscSUA-hscBA-fdx gene cluster and sufABCDSE operon, respectively. Another system, termed NIF (nitSU), is required for the maturation of nitrogenase in nitrogen-fixing bacteria. We have developed a novel genetic system to gain further insight into these multi-compo-nent systems, and to determine how ISC, SUP and NIF might differ in their roles in Fe-S assembly. We have constructed an E. coli mutant lacking both the isc and suf operons, and this strain can only survive in the presence of a complementing plasmid. Using the plasmid replacement technique, we examined the isc and suf operons, and identified the genes essential for the function. Additionally, we have found that nifSU-like genes cloned from Helicobacter pylori are functionally exchangeable with the isc and suf operons. Thus, the NIF-like system participates in the maturation of a wide variety of Fe-S proteins. An increased ability of NIF to complement isc and suf loss was seen under anaerobic conditions. This may explain why the NIF system is only found in a limited number of bacterial species, and most other organisms prefer the ISC and/or SUP systems. While the differences between ISC and SUF were small with respect to the complementing activity, the SUF system appears to be more advantageous for bacterial growth in the presence of hydrogen peroxide.

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