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The Redox-sensing Transcriptional Regulator RexT Controls Expression of Thioredoxin A2 in the Cyanobacterium Anabaena sp. Strain PCC 7120*

Background: Thioredoxins (Trxs) play a crucial role in the oxidative stress response. Results: A redox-sensing transcriptional regulator, RexT, controls expression of TrxA2, and TrxA2 regulates the DNA binding activity of RexT. Conclusion: The RexT-TrxA2 regulatory system regulates gene expression in response to redox state. Significance: This is the first report on a transcriptional regulator of the trx gene in cyanobacteria. Thioredoxins are ubiquitous proteins that catalyze thiol-disulfide redox reactions. They have a crucial role in the oxidative stress response as well as the redox regulation of metabolic enzymes. In cyanobacteria, little is known about the regulation of trx gene expression despite the importance of thioredoxins in cellular functions. In the present study, transcriptional regulation of the trx genes under oxidative stress conditions was investigated in the heterocystous cyanobacterium Anabaena sp. strain PCC 7120. When cells were exposed to H2O2, only the trxA2 gene (all1866) of seven trx genes was induced. Disruption of the rexT gene (alr1867), encoding a transcriptional regulator of the ArsR family, resulted in increased expression of trxA2. RexT bound to the region downstream of the transcription initiation site of trxA2. The DNA binding activity of RexT was impaired by H2O2 through the formation of an intramolecular disulfide bond, which induced expression of the trxA2 gene. The inactivated DNA binding activity of RexT was restored by reduced TrxA2. Hence, RexT is considered as a redox-sensing transcriptional repressor of trxA2. These results support the idea that the RexT-TrxA2 regulatory system is important for the oxidative stress response in this cyanobacterium.