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The biological synthesis of iron–sulfur (Fe–S) clusters requires dedicated pathways involved in the recruitment and activation of Fe and S for cluster assembly with subsequent transfer of preformed clusters to acceptor proteins. Several pathways have been described that include various numbers and types of biosynthetic components, although all of them share the same basic principles for [Fe–S] cluster formation and delivery to target proteins. The NifUS system was discovered and first described in studies involving the model diazotroph Azotobacter vinelandii . It has a dedicated role in serving as the starting point for the activation of [Fe–S] cluster-containing proteins specifically involved in biological nitrogen fixation. NifS is a pyridoxal-5′-phosphate containing l -cysteine-dependent sulfur transferase that delivers activated sulfur to the three-domain NifU, which not only serves as a scaffold for the construction of [2Fe–2S] and [4Fe–4S] clusters but also participates in their delivery to various target proteins involved in nitrogen fixation. Interestingly, analysis of sequenced genomes reveals that the three-domain NifU and NifU-like encoded proteins are not limited to diazotrophs, suggesting a broader role for this system in [Fe–S] cluster biogenesis in other organisms. The colocalization of adjacent nifU and nifS encoding sequences in most of these genomes also provides a strong indication for the involvement of the NifU–NifS [Fe–S] cluster assembly and delivery toolkit for activation of [Fe–S] cluster-containing proteins in a variety of organisms that do not fix nitrogen.