Accumulation of Cyanobacterial Photosystem II Containing the ‘Rogue’ D1 Subunit Is Controlled by FtsH Protease and Synthesis of the Standard D1 Protein

Abstract

Unicellular diazotrophic cyanobacteria contribute significantly to the photosynthetic productivity of the ocean and the fixation of molecular nitrogen, with photosynthesis occurring during the day and nitrogen fixation during the night. In species like Crocosphaera watsonii WH8501 the decline in photosynthetic activity in the night is accompanied by the disassembly of oxygen-evolving photosystem II (PSII) complexes. Moreover, in the second half of the night phase a small amount of rogue D1 (rD1), which is related to the standard form of D1 subunit found in oxygen-evolving PSII, but of unknown function, accumulates but is quickly degraded at the start of the light phase. We show here that removal of rD1 is independent of rD1 transcript level, thylakoid redox state and trans-thylakoidal pH but requires light and active protein synthesis. We also found that the maximal level of rD1 positively correlates with the maximal level of chlorophyll biosynthesis precursors and enzymes, which suggests a possible role for rPSII in the activation of chlorophyll biosynthesis just before or upon the onset of light, when new photosystems are synthesized. By studying strains of Synechocystis PCC 6803 expressing Crocosphaera rD1 we found that accumulation of rD1 is controlled by the light-dependent synthesis of the standard D1 protein which triggers the fast FtsH2-dependent degradation of rD1. Affinity purification of FLAG-tagged rD1 unequivocally demonstrated the incorporation of rD1 into a non-oxygen-evolving PSII complex which we term rogue PSII (rPSII). The complex lacks the extrinsic proteins stabilizing the oxygen-evolving Mn4CaO5 cluster but contains the Psb27 and Psb28-1 assembly factors.