Regulatory Role of LexA in Modulating Photosynthetic Redox Poise and Cadmium Stress Tolerance in the Cyanobacterium, Anabaena Sp. PCC7120

Strategies developed by organisms to overcome disruption in redox poise of photosynthetic electron transport chain (pETC) are important for its survival under abiotic stress. The process needs to be tightly regulated for optimal functioning. While the redox poising processes are well known in cyanobacteria, understanding of their regulatory network is lacking. Since LexA is one of the known global regulators of stress response in the cyanobacterium Anabaena sp. PCC7120, its role in pETC redox poising was investigated using cadmium (Cd) as an abiotic stressor to disrupt photosynthesis. Assessment of the photosynthetic responses of recombinant Anabaena strains, AnlexA+ (LexA-overexpressing) and AnpAM (vector control), under unstressed and Cd-stressed conditions using transmission electron microscopy (TEM) and chlorophyll a fluorescence, indicated that some pETC redox poising responses, including PSII photodamage, energy dissipation, PSI photoprotection, and NDH-mediated cyclic electron flow were decreased in AnlexA+ under unstressed conditions. Disturbance in pETC redox poise during Cd stress observed in Anabaena was accentuated upon overexpression of LexA. The decreased photodamage of PSII and increased photoinhibition of PSI in AnlexA+ in the presence or absence of Cd stress, correlated well with the changes in pETC complexes observed in blue native (BN)-PAGE and the regulation of over 70 of the 90 pETC component genes by LexA demonstrated through transcript, electromobility shift assay (EMSA), and bioinformatics studies. In a nutshell, LexA has been identified as one of the regulators involved in the streamlining of pETC redox poising responses under normal growth and during abiotic stress through transcriptional regulation of some of the redox-controlled pETC component genes.