Synthesis and turnover of photosystem II reaction center polypeptides in cyanobacterial D2 mutants.

Several mutations in psbD (coding for the photosystem II reaction protein D2) lead to a loss of the entire photosystem II complex from the thylakoid membrane. For two site-directed D2 mutants, E69V (Glu-69 mutated to Val) and ET-9 (introducing a premature stop codon in psbD), and two psbD deletion mutants, the synthesis and degradation of photosystem II reaction center components were analyzed. In E69V, no D2 or psbC-encoded CP43 was detected upon 5-min pulse labeling or Western blotting, whereas a psbDI/C transcript was present. Either D2 and CP43 are very unstable in this mutant, or the mutation causes an inhibition of translation of the psbDC operon. In E69V, the predominant lifetime of D1 is several minutes, but a small fraction of D1 remains stable in the thylakoid for several hours. In E69V, also a 43-kDa band is detected by D1 antisera. We attribute this band to a D1 cross-linking product possibly involved in D1 degradation. We identified a D1 precursor in E69V and psbDI/C/DII-, two mutants with particularly short D1 lifetimes. A unique feature of the E69V mutant is that destabilization of the photosystem II complex is triggered by a single-site mutation leading to extremely rapid turnover or lack of synthesis of D2.