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The Qo site of cytochrome b6f complexes controls the activation of the LHCII kinase

F Zito1, G Finazzi, R Delosme

  • 1UPR 1261 CNRS, Institut de Biologie Physico-Chimique, 13 rue Pierre et Marie Curie, 75005 Paris, France.

The EMBO Journal
|June 5, 1999
PubMed

Insights

Plastoquinol binding in the Qo pocket of cytochrome b6f is essential for activating light-harvesting complex II kinase. This process is crucial for state transitions in Chlamydomonas reinhardtii, enabling plants to adapt to changing light conditions.

Area of Science:

  • Photosynthesis research
  • Chloroplast biogenesis
  • Molecular plant biology

Background:

  • State transitions in photosynthesis involve dynamic rearrangements of light-harvesting complexes.
  • Cytochrome b6f complex plays a critical role in regulating these transitions.
  • Plastoquinol binding at the Qo site is a key regulatory step, but its precise function remains unclear.

Purpose of the Study:

  • To investigate the role of plastoquinol binding at the Qo site in activating light-harvesting complex II (LHCII) kinase.
  • To elucidate the mechanism by which cytochrome b6f regulates state transitions.

Main Methods:

  • Site-directed mutagenesis of the chloroplast petD gene to create a Qo pocket mutant (pwye) in Chlamydomonas reinhardtii.
  • Analysis of cytochrome b6f complex assembly and function.
  • Assessment of electron transfer rates and plastoquinol binding affinity.
  • In vivo 33Pi labeling and fluorescence measurements to monitor state transitions and LHCII kinase activity.

Main Results:

  • The pwye mutant assembled wild-type levels of cytochrome b6f but exhibited a complete block in electron transfer and lost plastoquinol binding at Qo.
  • Cells lacking plastoquinol binding remained locked in state I, showing no fluorescence quenching or LHCII kinase activation.
  • Antenna proteins in the mutant remained poorly phosphorylated, indicating a failure in state transition signaling.

Conclusions:

  • Plastoquinol binding within the Qo pocket of the cytochrome b6f complex is directly required for the activation of LHCII kinase.
  • This binding event is a critical prerequisite for initiating state transitions in Chlamydomonas reinhardtii.
  • The study provides direct evidence linking Qo site function to the regulation of photosynthetic light-harvesting adaptation.

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