Photosystem II (PSII), a light-driven water:
plastoquinone oxidoreductase, resides in the thylakoid membrane of prokaryotic
cyanobacteria and eukaryotic chloroplasts performing oxygenic photosynthesis.
The synthesis and assembly of a eukaryotic PSII require cooperation between the
chloroplast and nuclear genomes. Besides structural protein subunits, PSII
contains pigments and cofactors that are mainly ligated to its reaction center
proteins. Such an arrangement, together with the strongly oxidizing
photochemistry, renders the reaction center proteins vulnerable to photodamage
and accelerates their turnover. The primary target of photodamage is the D1
protein, which is constantly replaced in a dynamic process known as the PSII
photoinhibition repair cycle.