The thioredoxin reductase-glutaredoxins-ferredoxin crossroad pathway for selenate tolerance in Synechocystis PCC6803

Most organisms use two systems to maintain the redox homeostasis of cellular thiols. In the thioredoxin (Trx) system, NADPH sequentially reduces thioredoxin reductases (NTR), Trxs and protein disulfides. In the glutaredoxin (Grx) system, NADPH reduces the glutathione reductase enzyme occurring in most organisms, glutathione, Grxs, and protein disulfides or glutathione-protein mixed disulfides. As little is known concerning these enzymes in cyanobacteria, we have undertaken their analysis in the model strain Synechocystis PCC6803. We found that Grx1 and Grx2 are active, and that Grx2 but not Grx1 is crucial to tolerance to hydrogen peroxide and selenate. We also found that Synechocystis has no genuine glutathione reductase and uses NTR as a Grx electron donor, in a novel integrative pathway NADPH-NTR-Grx1-Grx2-Fed7 (ferredoxin 7), which operates in protection against selenate, the predominant form of selenium in the environment. This is the first report on the occurrence of a physical interaction between a Grx and a Fed, and of an electron transfer between two Grxs. These findings are discussed in terms of the (i) selectivity of Grxs and Feds (Synechocystis possesses nine Feds), (ii) crucial importance of NTR for cell fitness and (iii) resistance to selenate, in absence of a Thauera selenatis-like selenate reductase.