Structure and Energy Transfer Pathways in Dunaliella Photosystem I Supercomplex - increased complexity of PSI in a single organism along a novel cryo-EM processing method.

Photosystem I (PSI) has been at the focal point of photosynthetic research, due to its uncanny stability and a quantum yield approaching 100%. Though PSI is present in most photosynthetic organisms, its composition and oligomerization may vary drastically, depending on environmental conditions. The increased complexity of PSI became apparent from the high-resolution structures obtained for complexes isolated from various organisms, from cyanobacteria to plants. Comparison of structures from various eukaryotes revealed that despite having a similar gene composition, PSI can be assembled in different viable combinations. Using cryo-EM we were able to show two different assemblies of PSI in Dunaliella salina, which were obtained by slightly modifying our isolation procedures. The minimal composition consists of 7 core subunits – PsaA-PsaF and PsaJ, together with four bound light-harvesting complexes (LHCs). The supercomplex nearly doubled in size, and has six additional core subunits bound to it - PsaG, PsaH, PsaI, PsaK, PsaL and PsaO alongside two light harvesting complexes. One of the additional LHCs has a unique four helix structure which was observed only in the recently published Chlamydomonas reinhardtii PSI. Furthermore, we introduce a new method for cryo-EM single particle classification, where we treat each particle as its own class and improving its SNR by aligning and averaging it with several of its most similar particles. Using less than 4500 particles we were able to reconstruct an ab-initio low resolution model.

These findings serve as proof for the flexibility and dynamic nature of PSI.