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Stepwise Photoinhibition of Photosystem II1
Studies with Synechocystis Species PCC 6803 Mutants with a Modified D-E Loop of the Reaction Center Polypeptide D1

Paula Mulo, Susana Laakso, Pirkko Mäenpää, and Eva-Mari Aro*

Department of Biology, University of Turku, FIN-20014 Turku, Finland

Several mutant strains of Synechocystis sp. PCC 6803 with large deletions in the D-E loop of the photosystem II (PSII) reaction center polypeptide D1 were subjected to high light to investigate the role of this hydrophilic loop in the photoinhibition cascade of PSII. The tolerance of PSII to photoinhibition in the autotrophic mutant Delta R225-F239 (PD), when oxygen evolution was monitored with 2,6-dichloro-p-benzoquinone and the equal susceptibility compared with control when monitored with bicarbonate, suggested an inactivation of the QB-binding niche as the first event in the photoinhibition cascade in vivo. This step in PD was largely reversible at low light without the need for protein synthesis. Only the next event, inactivation of QA reduction, was irreversible and gave a signal for D1 polypeptide degradation. The heterotrophic deletion mutants Delta G240-V249 and Delta R225-V249 had severely modified QB pockets, yet exhibited high rates of 2,6-dichloro-p-benzoquinone-mediated oxygen evolution and less tolerance to photoinhibition than PD. Moreover, the protein-synthesis-dependent recovery of PSII from photoinhibition was impaired in the Delta G240-V249 and Delta R225-V249 mutants because of the effects of the mutations on the expression of the psbA-2 gene. No specific sequences in the D-E loop were found to be essential for high rates of D1 polypeptide degradation.

1   This study was financially supported by the Academy of Finland (Helsinki).
*   Corresponding author; e-mail evaaro{at}utu.fi; fax 358-2- 3338075.

Plant Physiol. (1998) 117: 483-490
Copyright Clearance Center:   0032-0889/98/117/0483/08
© 1998 American Society of Plant Physiologists




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