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Excessive CO₂ in the atmosphere is accelerating climate change, and plants (and algae) cannot adequately absorb these amounts. For this reason, research is being conducted on how to make photosynthesis – and thus CO₂ sequestration – more robust and efficient. However, this requires a precise understanding of its function and evolution. Dr. Jan Schuller from Philipps University of Marburg, working within the LOEWE Research Cluster RobuCop, has made a significant contribution to this field: Using high-resolution cryo-electron microscopy, he investigated the structure and evolution of a key step in photosynthesis.

The goal was to understand precisely how electrons are transferred between proteins during photosynthesis. In the alga Chlamydomonas reinhardtii, the protein cytochrome c₆ (Cyt c₆) is responsible for transporting electrons to photosystem I (PSI), while in plants, plastocyanin performs this role. From an evolutionary perspective, it is assumed that Cyt c₆ originally interacted with PSI and was later increasingly replaced by plastocyanin due to selective pressure. Even today, some cyanobacteria use only cytochrome c₆, some can switch between plastocyanin and Cyt c₆, and higher plants use only plastocyanin.

Investigating these processes has been difficult because the contact between the proteins is extremely short-lived. Cryo-electron microscopy has now enabled researchers to demonstrate in detail how the binding occurs: Negatively charged sites on Cyt c₆ are used for interaction with PSAF (a part of photosystem I) – similar to plastocyanin. Furthermore, the specific amino acid R66 – which is absent in plastocyanin but present in cyanobacteria – plays a central role in binding to and electron transfer from PSAF to PSI.

This study thus provides a rare insight into the interaction of ancestral proteins with photosystem I and offers a structural model for their evolution.