Figure 2 .

Gene clusters encoding bacterial Fe-S cluster biosynthetic systems: NIF, ISC, SUF, and SUF-like systems. All systems contain cysteine desulfurases, encoded by nifS, iscS, and sufS (yellow arrows). The nifU and iscU genes (red arrows) encode NifU and IscU, which are Fe-S cluster biosynthesis scaffold proteins in the NIF and ISC systems, respectively. In the SUF and SUF-like systems, a SufBCD protein complex encoded by sufB, sufC, and sufD (blue arrow) are regarded as a Fe-S cluster biosynthetic scaffold. The sufE gene (magenta arrow) and sufU gene (pink arrow) encode the sulfur-transfer proteins SufE and SufU, respectively. The other genes are not strictly essential under normal conditions, but become critical under certain growth conditions.