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The Exposed N-Terminal Tail of the D1 Subunit Is Required for Rapid D1 Degradation during Photosystem II Repair in Synechocystis sp PCC 6803^[W]

Josef Komenda^a,b,1, Martin Tich^a,b, Ondej Práil^a,b, Jana Knoppová^a,b, Stanislava Kuviková^a,b, Remco de Vries^c and Peter J. Nixon^c

^a Institute of Microbiology, Academy of Sciences, Opatovick mln, 37981 Tebo, Czech Republic
^b Institute of Physical Biology, University of South Bohemia, Zámek 136, 37333 Nové Hrady, Czech Republic
^c Division of Biology, Faculty of Natural Sciences, Imperial College London, London SW7 2AZ, United Kingdom

¹ Address correspondence to komenda{at}alga.cz.

The selective replacement of photodamaged D1 protein within the multisubunit photosystem II (PSII) complex is an important photoprotective mechanism in chloroplasts and cyanobacteria. FtsH proteases are involved at an early stage of D1 degradation, but it remains unclear how the damaged D1 subunit is recognized, degraded, and replaced. To test the role of the N-terminal region of D1 in PSII biogenesis and repair, we have constructed mutants of the cyanobacterium Synechocystis sp PCC 6803 that are truncated at the exposed N terminus. Removal of 5 or 10 residues blocked D1 synthesis, as assessed in radiolabeling experiments, whereas removal of 20 residues restored the ability to assemble oxygen-evolving dimeric PSII complexes but inhibited PSII repair at the level of D1 degradation. Overall, our results identify an important physiological role for the exposed N-terminal tail of D1 at an early step in selective D1 degradation. This finding has important implications for the recognition of damaged D1 and its synchronized replacement by a newly synthesized subunit.