Abstract
Photosystem I (PSI) is the dominant photosystem in cyanobacteria and it plays a pivotal role in cyanobacterial metabolism. Despite its biological importance, the native organization of PSI in cyanobacterial thylakoid membranes is poorly understood. Here, we use atomic force microscopy (AFM) to show that ordered, extensive macromolecular arrays of PSI complexes are present in thylakoids from Thermosynechococcus elongatus, Synechococcus sp PCC 7002, and Synechocystis sp PCC 6803. Hyperspectral confocal fluorescence microscopy and three-dimensional structured illumination microscopy of Synechocystis sp PCC 6803 cells visualize PSI domains within the context of the complete thylakoid system. Crystallographic and AFM data were used to build a structural model of a membrane landscape comprising 96 PSI trimers and 27,648 chlorophyll a molecules. Rather than facilitating intertrimer energy transfer, the close associations between PSI primarily maximize packing efficiency; short-range interactions with Complex I and cytochrome b6f are excluded from these regions of the membrane, so PSI turnover is sustained by long-distance diffusion of the electron donors at the membrane surface. Elsewhere, PSI-photosystem II contact zones provide sites for docking phycobilisomes and the formation of megacomplexes. PSI-enriched domains in cyanobacteria might foreshadow the partitioning of PSI into stromal lamellae in plants, similarly sustained by long-distance diffusion of electron carriers.
Highlights
Photosystem I (PSI) is one of the two pigment-protein complexes that underpin photosynthesis in cyanobacteria, algae, and plants
Given that PSI is the major photosystem complex in many cyanobacteria, this study investigates its membrane organization in T. elongatus, Synechococcus 7002, and Synechocystis using atomic force microscopy (AFM), hyperspectral confocal fluorescence microscopy (HCFM), and three-dimensional structured illumination microscopy (3D-SIM)
We found that 1.0% (w/w) digitonin, a concentration 10-fold higher than that used to prepare membranes for AFM imaging, did not affect the PSI:photosystem II (PSII) ratio, which remained at ;4 (Supplemental Figure 3D)
Summary
Photosystem I (PSI) is one of the two pigment-protein complexes that underpin photosynthesis in cyanobacteria, algae, and plants. The thermophilic cyanobacteria Thermosynechococcus elongatus and T. vulcanus are significant, as they are the source of purified, stable complexes for a series of x-ray crystallography studies that determined the structures of PSI (Jordan et al, 2001) and PSII (Ferreira et al, 2004; Loll et al, 2005; Umena et al, 2011; Suga et al, 2015) Other cyanobacteria such as Synechococcus sp PCC 7002 (hereafter Synechococcus 7002) and Synechocystis sp PCC 6803 (hereafter Synechocystis) are heavily used models for many areas of research, for example, photosynthesis, gene regulation, and synthetic biology. Given that PSI is the major photosystem complex in many cyanobacteria, this study investigates its membrane organization in T. elongatus, Synechococcus 7002, and Synechocystis using atomic force microscopy (AFM), hyperspectral confocal fluorescence microscopy (HCFM), and three-dimensional structured illumination microscopy (3D-SIM)
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