Crystal structures of photosystem II from a cyanobacterium expressing psbA in comparison to psbA reveal differences in the D1 subunit.

Three psbA genes (psbA, psbA, and psbA) encoding the D1 subunit of photosystem II (PSII) are present in the thermophilic cyanobacterium Thermosynechococcus elongatus and are expressed differently in response to changes in the growth environment. To clarify the functional differences of the D1 protein expressed from these psbA genes, PSII dimers from two strains, each expressing only one psbA gene (psbA or psbA), were crystallized, and we analyzed their structures at resolutions comparable to previously studied PsbA1-PSII. Our results showed that the hydrogen bond between pheophytin/D1 (Pheo) and D1-130 became stronger in PsbA2- and PsbA3-PSII due to change of Gln to Glu, which partially explains the increase in the redox potential of Pheo observed in PsbA3. In PsbA2, one hydrogen bond was lost in Pheo due to the change of D1-Y147F, which may explain the decrease in stability of Pheo in PsbA2. Two water molecules in the Cl-1 channel were lost in PsbA2 due to the change of D1-P173M, leading to the narrowing of the channel, which may explain the lower efficiency of the S-state transition beyond S in PsbA2-PSII. In PsbA3-PSII, a hydrogen bond between D1-Ser270 and a sulfoquinovosyl-diacylglycerol molecule near Q disappeared due to the change of D1-Ser270 in PsbA1 and PsbA2 to D1-Ala270. This may result in an easier exchange of bound Q with free plastoquinone, hence an enhancement of oxygen evolution in PsbA3-PSII due to its high Q exchange efficiency. These results provide a structural basis for further functional examination of the three PsbA variants.