Isolation of a novel heterodimeric PSII complex involved in PSII repair

The multi-subunit membrane protein complex Photosystem II (PSII) catalyzes the light driven oxidation of water and with this the initial step of photosynthetic electron transport in plants, algae, and cyanobacteria. Its biogenesis is coordinated by a network of auxiliary proteins that facilitate the stepwise assembly of individual subunits and cofactors, forming various intermediate complexes until fully functional mature PSII is present at the end of the process. In the current study, we purified PSII complexes from a mutant line of the thermophilic cyanobacterium Thermosynechococcus elongatus in which the extrinsic subunit PsbO, characteristic for active PSII, was fused with an N-terminal TwinStrep-Tag. Three distinct PSII complexes were separated by ion exchange chromatography after the initial affinity purification. Two complexes differ in their oligomeric state (monomeric and dimeric) but share the typical subunit composition of mature PSII. They are characterized by very high oxygen evolving activity of approx. 6,000 µmol O2· (mg Chl·h)-1. Analysis of the third (heterodimeric) PSII complex revealed lower oxygen evolving activity of approx. 3000 µmol O2· (mg Chl·h)-1 and a manganese content of 2.7 (± 0.2) per reaction center compared to 3.7 (± 0.2) of fully active PSII. Mass spectrometry and time-resolved fluorescence spectroscopy further indicated that PsbO is partially replaced by Psb27 in this PSII fraction, thus implying a role in repair of the complex.