The oxygen-evolving photosystem II (PSII) complex located in
chloroplasts and cyanobacteria is sensitive to light-induced
damage1 that unless repaired causes reduction in photosynthetic
capacity and growth. Although a potential target for
crop improvement, the mechanism of PSII repair remains
unclear. The D1 reaction center protein is the main target for
photodamage2, with repair involving the selective degradation
of the damaged protein by FtsH protease3. How a single
damaged PSII subunit is recognized for replacement is
unknown. Here, we have tested the dark stability of PSII subunits
in strains of the cyanobacterium Synechocystis PCC
6803 blocked at specific stages of assembly. We have found
that when D1, which is normally shielded by the CP43
subunit, becomes exposed in a photochemically active PSII
complex lacking CP43, it is selectively degraded by FtsH even
in the dark. Removal of the CP47 subunit, which increases
accessibility of FtsH to the D2 subunit, induced dark degradation
of D2 at a faster rate than that of D1. In contrast,
CP47 and CP43 are resistant to degradation in the dark. Our
results indicate that protease accessibility induced by PSII disassembly
is an important determinant in the selection of the D1
and D2 subunits to be degraded by FtsH.