Lambrev, P. H. | bioRxiv Science

bioRxiv2023

Accumulation and light-harvesting function of IsiA in cyanobacterial cells with monomeric and trimeric Photosystem I

The acclimation of cyanobacteria to iron deficiency is crucial for their survival in natural environments. In response to iron deficiency, many cyanobacterial species induce the production of a pigment-protein complex called IsiA. IsiA proteins associate with photosystem I (PSI) and can function as light-harvesting antennas or dissipate excess energy. They may also serve as Chl storage during iron limitation. In this study we examined the functional role of IsiA in cells of Synechocystis sp. PCC 6803 grown under iron limitation conditions by measuring the cellular IsiA content and its capability to transfer energy to PSI. We specifically test the effect of the oligomeric state of PSI by comparing wild-type (WT) Synechocystis sp. PCC 6803 to mutants lacking specific subunits of PSI, namely PsaL/PsaI ({Delta}psaL mutant) and PsaF/PsaJ ({Delta}FIJL). Time-resolved fluorescence spectroscopy revealed that IsiA formed functional PSI3-IsiA18 supercomplexes, wherein IsiA effectively transfers energy to PSI on a timescale of 10 ps at room temperature - measured in isolated complexes and in vivo - confirming the primary role of IsiA as an accessory light-harvesting antenna to PSI. However, a significant fraction (40%) remained unconnected to PSI, supporting the notion of a dual functional role of IsiA. Cells with monomeric PSI under iron deficiency contained only 3-4 IsiA complexes bound to PSI. Together the results show that IsiA is capable of transferring energy to trimeric and monomeric PSI but to varying degrees and that the acclimatory production of IsiA under iron stress is controlled by its ability to perform its light-harvesting function.

biophysics↗

bioRxiv2021

Trimeric Photosystem I facilitates energy transfer from phycobilisomes in Synechocystis PCC 6803

In cyanobacteria, phycobilisomes serve as peripheral light-harvesting complexes of the two photosystems, extending their antenna size and the wavelength range of photons available for photosynthesis. The abundance of phycobilisomes, the number of phycobiliproteins they contain, and their light-harvesting function are dynamically adjusted in response to the physiological conditions. Phycobilisomes are also thought to be involved in state transitions that maintain the excitation balance between the two photosystems. Unlike its eukaryotic counterpart, PSI is trimeric in many cyanobacterial species and the physiological significance of this is not well understood. Here we compared the composition and light-harvesting function of phycobilisomes in cells of Synechocystis sp. PCC 6803, which has primarily trimeric PSI, and the{Delta} psaL mutant unable to form trimers. We also investigated a mutant additionally lacking the PsaJ and PsaF subunits of PSI. Both strains with monomeric PSI accumulated significantly more allophycocyanin per chlorophyll, indicating higher abundance of phycobilisomes. On the other hand, a higher phycocyanin:allophycocyanin ratio in WT suggests larger phycobilisomes or the presence of APC-less phycobilisomes (CpcL-type), that are not assembled in cells with monomeric PSI. Steady-state and time-resolved fluorescence spectroscopy at room temperature and 77 K revealed that PSII receives more energy from the phycobilisomes at the expense of PSI in cells with monomeric PSI, regardless of the presence of PsaF. Taken together, these results show that the oligomeric state of PSI has an impact on the excitation energy flow in Synechocystis. One-sentence summaryCyanobacterial mutants with monomeric PSI show changes in the composition and abundance of phycobilisomes and in the excitation energy transfer to PSII and PSI.

biophysics↗