Abstract
ABSTRACTOxygenic photosynthetic organisms have evolved a multitude of mechanisms for protection against high-light stress. IsiA, a chlorophyll a-binding cyanobacterial protein, serves as an accessory antenna complex for photosystem I. Intriguingly, IsiA can also function as an independent pigment protein complex in the thylakoid membrane and facilitate the dissipation of excess energy, providing photoprotection. The molecular basis of the IsiA-mediated excitation quenching mechanism remains poorly understood. In this study, we demonstrate that IsiA uses a novel cysteine-mediated process to quench excitation energy. The single cysteine in IsiA in the cyanobacterium Synechocystis sp. strain PCC 6803 was converted to a valine. Ultrafast fluorescence spectroscopic analysis showed that this single change abolishes the excitation energy quenching ability of IsiA, thus providing direct evidence of the crucial role of this cysteine residue in energy dissipation from excited chlorophylls. Under stress conditions, the mutant cells exhibited enhanced light sensitivity, indicating that the cysteine-mediated quenching process is critically important for photoprotection.
Highlights
Oxygenic photosynthetic organisms have evolved a multitude of mechanisms for protection against high-light stress
iron stress-induced protein A (IsiA) increases the effective absorption cross section of photosystem I (PSI) by ;60% [24]. These results showed that in the PSI3-IsiA18 supercomplex, IsiA serves as an accessory antenna for PSI
The mechanism of this process has been carefully dissected during recent years [39, 40]. We have described another novel and potentially more universal mechanism of photoprotection based on excitation quenching mediated by a Cys residue in the Chl antenna protein IsiA
Summary
Oxygenic photosynthetic organisms have evolved a multitude of mechanisms for protection against high-light stress. Cyanobacteria have developed a number of strategies to modulate light absorption and dissipation to ensure maximal photosynthetic productivity and minimal photodamage to cells under extreme light conditions In this communication, we have determined the critical role of a novel cysteine-mediated mechanism for light energy dissipation in the chlorophyll protein IsiA. In addition to the PSI3-IsiA18 supercomplex, IsiA-only pigment protein complexes have been observed in cyanobacterial cells after prolonged growth under irondepleted conditions [25] as well as under high-light conditions [15] Such IsiA-only complexes have been suggested to be involved in nonphotochemical quenching processes [26]. Such a mechanism was originally proposed for the Fenna-Matthews-Olson (FMO) protein complex in anoxygenic green sulfur bacteria [32]
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