Understanding Excitation Energy Quenching in IsiA
Permanent URL: https://doi.org/10.7936/drjx-7778
Cyanobacteria are photoautotrophic organisms that contribute a significant amount of global primary productivity. They are found in freshwater, marine and even some extremely severe environments. Among those environments, iron deficiency is one of the most common stress conditions in cyanobacterial habitats. To survive, cyanobacteria have evolved and developed several strategies to alleviate the damage caused by iron deficiency. Iron stress-inducible protein (IsiA) is a chlorophyll-binding membrane protein found in cyanobacteria grown in iron-deficient conditions. During the past decades, considerable effort has been put on understanding how IsiA functions to help cyanobacteria survive iron deficiency. It has been reported that IsiA forms various ring-shaped complexes with PSI (PSIx-IsiAy) or by itself (IsiA aggregate). While coupled with PSI (PSIx-IsiAy), the IsiA protein serves as an accessary antenna for PSI, which increases the absorption cross-section by 60% compared with the PSI trimer. IsiA aggregate, instead, dissipates excess light energy to prevent the cells from photodamage. Although these functions have been discovered and demonstrated in vivo and in vitro, the detailed mechanisms, especially the non-photochemical quenching process in IsiA, were not well understood. In this study, the excitation energy quenching process in IsiA was investigated by time-resolved spectroscopy, and we proposed that IsiA dissipate excitation energy via a cysteine-mediated quenching process. Site-directed mutagenesis was performed to replace this critical cysteine (C260) in IsiA with a valine. This single amino acid substitution in IsiA results in the defective IsiA that no longer quenches excitation energy but still functions as light-harvesting antenna for PSI. Interestingly, this IsiA mutant grew faster than the wild type in the presence of iron under high light, suggesting a more efficient use of light energy due to the abolishment of a photoprotective mechanism. Sharing a similar structure with IsiA, CP43, an intrinsic antenna of PSII, has not been reported being involved in excitation energy quenching process. We attempted to introduce this cysteine-mediated quenching process into CP43 to directly provide photoprotection to the reaction center of PSII. The mutant CP43 phenotypes showed a 25% lower quantum efficiency of PSII and barely grew photoautotrophically, implying an inefficient energy transfer to the reaction center of PSII caused by the introduction of an extrinsic quenching process.