Purification of a minimal complex for oxygen evolution from the recombinant cyanobacteria that carry thermostable photosystem II

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Photosystem II reaction center subunits, D1-D2 heterodimer catalyze water splitting and oxygen evolving reaction. To establish a method to isolate the heterodimer, D1 and D2 proteins from a thermophilic cyanobacterium Thermasynechococcus vulcanus were expressed in a mesophilic Synechococcus elongatus by constructing the recombinant S.elongatus via rps12-mediated gene replacement. A GRPS1 carrying an rps12R-43 mutation was transformed to obtain GRPS201 and GRPS220 strains. GRPS201 carries a replaced psbA1 gene, whereas GRPS220 carries both replaced psbA1 and psbD1 genes from T.vulcanus Thylakoid membrane proteins were prepared from these cyanobacteria, and analyzed by Western blotting using an antibody specific for T.vulcanusD1-1 protein. Although D1 protein was detected in GRPS201 and GRPS220 strains, the size of D1-1 was larger by 2kDa than the native D1-1 from T.vulcanus, suggesting that C-terminal 16 amino acids were not processed. On the other hand, T.vulcanus HspA chaperon protein was purified, which would be useful for removing aggregated proteins arising from the heat-treated thylakoid membrane to obtain the purified D1-D2 heterodimer proteins.