Interaction of the GntR-family transcription factor Sll1961 with thioredoxin in the cyanobacterium Synechocystis sp. PCC 6803

Changes in the redox state of the photosynthetic electron transport chain act as a signal to trigger acclimation responses to environmental cues and thioredoxin has been suggested to work as a key factor connecting the redox change with transcriptional regulation in the cyanobacterium Synechocystis sp. PCC 6803. We screened for redox-dependent transcription factors interacting with thioredoxin M (TrxM) and isolated the GntR-type transcription factor Sll1961 previously reported to be involved in acclimation responses of the photosynthetic machinery. Biochemical analyses using recombinant Sll1961 proteins of wild type and mutants of three cysteine residues, C124, C229 and C307, revealed that an intramolecular disulfide bond is formed between C229 and C307 under oxidizing conditions and TrxM can reduce it by attacking C307. Sll1961 exists in a dimeric form of about 80 kDa both under reducing and oxidizing conditions. C124 can form an intermolecular disulfide bond but it is not essential for dimerization. Based on these observations, tertiary structure models of the Sll1961 homodimer and the Sll1961-TrxM complex were constructed.