Thiol redox switches regulate the oligomeric state of cyanobacterial Rre1, RpaA and RpaB response regulators

Cyanobacteria employ two‐component sensor‐response regulator systems to monitor and respond to environmental challenges. The response regulators RpaA, RpaB, Rre1 and RppA are integral to circadian clock function and abiotic stress acclimation in cyanobacteria. RpaA, RpaB and Rre1 are known to interact with ferredoxin or thioredoxin, raising the possibility of their thiol regulation. Here, we report that Synechocystis sp. PCC 6803 Rre1, RpaA and RpaB exist as higher‐order oligomers under oxidising conditions and that reduced thioredoxin A converts them to monomers. We further show that these response regulators contain redox‐responsive cysteine residues with an Em7 around −300 mV. These findings suggest a direct thiol modulation of the activity of these response regulators, independent of their cognate sensor kinases. We show thiol‐mediated redox regulation of cyanobacterial response regulators Rre1, RpaA and RpaB, which are involved in circadian clock function and abiotic stress acclimation. Oxidative stress causes these response regulators to oligomerise independently of phosphorylation, adding another layer of regulation to their DNA‐binding activity. Reduced thioredoxin A converts the higher‐order oligomers of these response regulators to monomers.