Unraveling the Redox Properties of the Global Regulator FurA fromAnabaenasp. PCC 7120: Disulfide Reductase Activity Based on Its CXXC Motifs

Abstract

Cyanobacterial FurA works as a global regulator linking iron homeostasis to photosynthetic metabolism and the responses to different environmental stresses. Additionally, FurA modulates several genes involved in redox homeostasis and fulfills the characteristics of a heme-sensor protein whose interaction with this cofactor negatively affects its DNA binding ability. FurA from Anabaena PCC 7120 contains five cysteine residues, four of them arranged in two redox CXXC motifs. Our goals were to analyze in depth the putative contribution of these CXXC motifs in the redox properties of FurA and to identify potential interacting partners of this regulator. Insulin reduction assays unravel that FurA exhibits disulfide reductase activity. Simultaneous presence of both CXXC signatures greatly enhances the reduction rate, although the redox motif containing Cys(101) and Cys(104) seems a major contributor to this activity. Disulfide reductase activity was not detected in other ferric uptake regulator (Fur) proteins isolated from heterotrophic bacteria. In vivo, FurA presents different redox states involving intramolecular disulfide bonds when is partially oxidized. Redox potential values for CXXC motifs, -235 and -238 mV, are consistent with those reported for other proteins displaying disulfide reductase activity. Pull-down and two-hybrid assays unveil potential FurA interacting partners, namely phosphoribulokinase Alr4123, the hypothetical amidase-containing domain All1140 and the DNA-binding protein HU. A novel biochemical activity of cyanobacterial FurA based on its cysteine arrangements and the identification of novel interacting partners are reported. The present study discloses a putative connection of FurA with the cyanobacterial redox-signaling pathway.