Mutagenesis of Ile184 in the cd-loop of the photosystem II D1 protein modifies acceptor-side function via spontaneous mutation of D1-His252 in Synechocystis sp. PCC 6803

The Photosystem II water-plastoquinone oxidoreductase is a multi-subunit complex which catalyses the lightdriven oxidation of water to molecular oxygen in oxygenic photosynthesis. The D1 reaction centre protein exists in multiple forms in cyanobacteria, including D1FR which is expressed under far-red light. We investigated the role of Phe184 that is found in the lumenal cd-loop of D1FR but is typically an isoleucine in other D1 isoforms. The I184F mutant in Synechocystis sp. PCC 6803 was similar to the control strain but accumulated a spontaneous mutation that introduced a Gln residue in place of His252 located on the opposite side of the thylakoid membrane. His252 participates in the protonation of the secondary plastoquinone electron acceptor QB. The I184F: H252Q double mutant exhibited reduced high-light-induced photodamage and an altered QB-binding site that impaired herbicide binding. Additionally, the H252Q mutant had a large increase in the variable fluorescence yield although the number of photochemically active PS II centres was unchanged. In the I184F:H252Q mutant the extent of the increased fluorescence yield decreased. Our data indicates substitution of Ile184 to Phe modulates PS II-specific variable fluorescence in cells with the His252 to Gln substitution by modifying the QBbinding site.