Molecular and functional insights into glutathione S-transferase genes associated with salt stress in Halothece sp. PCC7418

Evolution and function of glutathione S-transferase (GST) in primordial oxygenic phototrophs such as cyanobacteria are poorly understood. In this study, we identified and functionally characterized the GST gene family in the halotolerant cyanobacterium Halothece sp. PCC7418. Four putative Halothece-GSTs had very low homology, which implies evolutionary divergence. Of these, H0647, H0729 and H3557 were differentially expressed by oxidative stress whereas H3557 was highly and specifically upregulated under salt stress. In vitro analysis revealed that the recombinant H3557 exhibited GST activity toward 1-chloro-2, 4-dinitrobenzene (CDNB) and glutathione (GSH). H3557 displayed a broad range of activity at pH 6.5-10.5. Kinetic parameters showed the apparent K_m for CDNB and GSH was 0.14 and 0.75 mM, respectively. H3557 remained catalytically active in the presence of NaCl. Structural modelling supported that H3557 is salt-adaptive enzyme with highly acidic residues on the protein surface. The vital function of H3557 in heterologous expression system was evaluated. The H3557-expressing cells were more tolerant to H₂ O₂ -induced oxidative stress compared with other GST-expressing cells and conferred salt tolerance. Taken together, the findings of this study provide insights into the molecular and cellular functions of GST in cyanobacteria, particularly under salt stress, which is less understood compared with other species.