The Effect of Light Intensity on Phycobilisome Complexes’ Architecture and Composition

 

  1. Suissa Szlejf1 and N. Adir1
    1Schulich Faculty of Chemistry, Technion, Haifa 32000, Israel

sumaayan@campus.technion.ac.il

 

Photosynthesis is the biological process of converting light energy into chemical energy. Organisms such as plants, algae, and cyanobacteria utilize this process to sustain life and carry out many cellular processes. Cyanobacterial cells contain unique membranes, housing the photosynthetic system. The system is composed of many protein complexes, where the two key components that drive the process are antenna complexes and reaction centers (RC). The antenna complexes are also called light harvesting complexes (LHC), corresponding to their function. Upon light irradiation, light energy is absorbed by the LHCs and is immediately transferred to an RC, where the chemical and electron transfer reactions occur. The energy transfer from the LHC to the RC is highly efficient and relies on an overlap between the energy emitted by the LHC and that absorbed by the RC. The current mechanisms do not fully explain the excitation energy transfer (EET) occurring inside and between the complexes. In this research, we investigate EET in a unique cyanobacteria, A. marina, where there is a large energetic and physical gap between LHC and RC, yet it remains efficient, even under varying light intensities. In practice, we utilize a unique phenomenon found only in the A. marina membranes, where there is a heterogeneous separation between domains that contain the main LHC (the Phycobilisome) and domains lacking this complex. These unique domains, which we call LR-islands, include all the components of the photosynthetic system, which poses an opportunity to investigate the system in its entirety. We have found that in different light intensities, the organism changes the composition of the thylakoid membranes. In low light, there is an increase in A. marina Phycobilisome (AmPBS) production and the complex is present in all the regions, as opposed to high light where the unique LR-islands appear. We investigate particularly the AmPBS complex, known to contain only phycocyanin (PC), assembled from two isoforms of the α and β subunits, in varying ratios. Using structural tools such as crystallography and CryoEM, in addition to spectroscopic and analytical tools, we can shed more light on the role of each isoform within the phycobilisome. Here we present the structure of AmPBS complex in low-light conditions, with linkers spread evenly inside it, to create a symmetrical structure between the membranes. We also present a structure of an AmPC type which is proposed to act as a terminal emitter, with red-shifted emission.

Additionally, in collaboration with Sui group (Tsinghua University) and the Keren and Paltiel groups (HUJI), we have studied the effects of light intensity on the PBS complex from red algae P. purpureum. Following a comparison between the low-light structure of the PBS (published in 2020) and the medium-light structure of the PBS (our present work), we have found major differences in the subunit composition, as well as the pigments’ locations and quantity. This research elucidates the ability of fast acclimation by an organism which resides in a changing environment and may help in understanding other acclimation mechanisms.

Finally, in collaboration with Beja and Kleifeld groups (Technion), we have studied the effects of a disassembly-related protein called NblA, on the composition and assembly level of the PBS complex during starvation conditions, both in freshwater and marine cyanobacteria. We propose a mechanism of specificity, by which these NblAs are used, according to each organism.