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PGR5 is a thylakoid-associated protein implicated in regulation of cyclic electron transport around photosystem I in plants. Unlike linear electron transport, a cyclic route can contribute to a proton-motive force without producing the same net output of reducing equivalents. This helps adjust the balance between ATP supply and other photosynthetic demands. ^[ref]

The original Arabidopsis thaliana mutant failed to downregulate photochemistry normally under intense illumination. The proposed connection was that altered electron-flow regulation weakens the pH difference that supports protective energy dissipation. The genetic result establishes a functional requirement in the pathway; it does not by itself show that isolated PGR5 is the complete electron-transfer catalyst.

Researcher in photosynthesis. Munekage and collaborators investigated PGR5 in cyclic electron flow around photosystem I and its role in photoprotection, linking electron routing with tolerance of excess light.^[ref]