In vivo [Fe-S] cluster acquisition by IscR and NsrR, two stress regulators in Escherichia coli

The multi-proteins Isc and Suf systems catalyse the biogenesis of [Fe-S] proteins. Here we investigate how NsrR and IscR, transcriptional regulators that sense NO and [Fe-S] homeostasis, acquire their [Fe-S] clusters under both normal and iron limitation conditions. Clusters directed at the apo-NsrR and apo-IscR proteins are built on either of the two scaffolds, IscU or SufB. However, differences arise in [Fe-S] delivery steps. In the case of NsrR, scaffolds deliver clusters to either one of the two ATCs, IscA and SufA, and, subsequently, to the non-Isc non-Suf ATC, ErpA. Nevertheless, a high level of SufA can bypass the requirement for ErpA. In the case of IscR, several routes occur. One does not include assistance of any ATC. Others implicate ATCs IscA or ErpA, but, surprisingly, SufA was totally absent from any IscR maturation pathways. Both IscR and NsrR have the intrinsic capacity to sense iron limitation. However, NsrR appeared to be efficiently matured by Isc and Suf, thereby preventing NsrR to act as a physiologically relevant iron sensor. This work emphasizes that different maturation pathways arise as a function of the apo-target considered, possibly in relation with the type of cluster, [2Fe-2S] versus [4Fe-4S], it binds.