Proposed IrrB-regulated networks in MSR-1 cells. Under high-iron conditions, IrrB indirectly regulates genes for heme biosynthesis (hemA, hemB, hemC), iron storage (bfrA, bfrB), and an unknown pathway. Under low-iron conditions, IrrB regulates genes for iron transport (bhuA), Fe-S cluster assembly (sufA), and nitrogenase metallocluster biosynthesis (nifS, nifU).