FIG 7.

Proposed model of SufR-mediated Fe-S cluster biogenesis under normal and oxidative stress conditions in S. avermitilis. Under normal conditions, the Fe-S clusters (crimson rhombuses) synthesized by the Suf system are sufficient to satisfy the requirement of intracellular Fe-S cluster proteins. SufR exists mainly in the form of [4Fe-4S] SufR and represses transcription of the suf operon. Under oxidative stress, destruction of Fe-S clusters results in an increased demand for intracellular Fe-S clusters. Apo-SufR loses its ability to bind to the sufR promoter region, which leads to derepression of the suf operon and therefore to increased synthesis of Fe-S clusters.