Functional Significance of NADPH-Thioredoxin Reductase C in the Antioxidant Defense System of Cyanobacterium Anabaena sp. PCC 7120.

The redox regulation system is widely accepted as a crucial mechanism for controlling the activities of various metabolic enzymes. In addition to thioredoxin reductase/thioredoxin cascades, NADPH-thioredoxin reductase C (NTRC), a hybrid protein formed by an NADPH-thioredoxin reductase domain and a thioredoxin (Trx) domain, is present in chloroplasts and in most cyanobacteria species. Although several target proteins and physiological functions of NTRC in chloroplasts have been characterized, little is known about NTRC functions in cyanobacteria. Therefore, we investigated the molecular basis and physiological significance of NTRC-dependent redox regulation in the filamentous heterocyst-forming nitrogen-fixing cyanobacterium Anabaena sp. PCC 7120 (Anabaena 7120). Initially, we identified six candidate NTRC targets in Anabaena 7120 using NTRC affinity chromatography. Subsequently, we compared the efficiency of reducing-equivalent transfer from NTRC and Trx-m1 to the NTRC target protein 2-Cys peroxiredoxin. Biochemical analyses revealed that compared with Trx-m1, NTRC more efficiently transfers reducing equivalents to 2-Cys peroxiredoxin. Subsequently, we constructed and analyzed an ntrC knockout strain in Anabaena 7120. The mutant showed impaired growth under oxidative stress conditions and lower concentrations of reduced 2-Cys peroxiredoxin in cells. Taken together, the present in vitro and in vivo results indicate that NTRC is a significant electron donor for 2-Cys peroxiredoxin and plays a pivotal role in antioxidant defense systems in Anabaena 7120 cells.