Modification of the pheophytin redox potential in Thermosynechococcus elongatus Photosystem II with PsbA3 as D1.

In Photosystem II (PSII) of the cyanobacterium Thermosynechococcus elongatus, glutamate 130 in the high-light variant of the D1-subunit (PsbA3) was changed to glutamine in a strain lacking the two other genes for D1, psbA1 and psbA2. The resulting PSII (PsbA3/Glu130Gln) was compared with those from the "native" high-light (PsbA3-PSII) and low-light (PsbA1-PSII) variants, which differ by 21 amino acid including Glu130Gln. H-bonding from D1-Glu130Gln to the primary electron acceptor, PheophytinD1 (PheoD1), is known to affect the Em of the PheoD1/PheoD1(-) couple. The Gln130 mutation here had little effect on water splitting, charge accumulation and photosensitivity but did slow down S2QA(-) charge recombination and up-shift the thermoluminescence while increasing its yield. These changes were consistent with a ≈-30mV shift of the PheoD1/PheoD1(-)Em, similar to earlier single site-mutation results from other species and double the ≈-17mV shift seen for PsbA1-PSII versus PsbA3-PSII. This is attributed to the influence of the other 20 amino-acids that differ in PsbA3. A computational model for simulating S2QA(-) recombination matched the experimental trend: the S2QA(-) recombination rate in PsbA1-PSII differed only slightly from that in PsbA3-PSII, while in Glu130-PsbA3-PSII there was a more pronounced slowdown of the radical pair decay. The simulation predicted a major effect of the PheoD1/PheoD1(-) potential on (1)O2 yield (~60% in PsbA1-PSII, ~20% in PsbA3-PSII and ~7% in Gln130-PsbA3-PSII), reflecting differential sensitivities to high light.

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