Characterization of a redox-active cross-linked complex between cyanobacterial photosystem I and its physiological acceptor flavodoxin

Ulrich Mühlenhoff, Jochen Kruip, Donald A. Bryant, Matthias Rögner, Pierre Sétif, Egbert Boekema

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Abstract

A covalent complex between photosystem I and flavodoxin from the cyanobacterium Synechococcus sp. PCC 7002 was generated by chemical cross-linking. Laser flash-absorption spectroscopy indicates that the bound flavodoxin of this complex is stabilized in the semiquinone state and is photoreduced to the quinol form upon light excitation. The kinetics of this photoreduction process, which takes place in ~50% of the reaction centres, displays three exponential components with half-lives of 9 µs, 70 µs and 1 ms. The fully reduced flavodoxin subsequently recombines with P700+ with a t1/2 of 330 ms. A corresponding flavodoxin semiquinone radical signal is readily observed in the dark by room temperature electron paramagnetic resonance, which reversibly disappears upon illumination. In contrast, the light-induced reduction of oxidized flavodoxin can be observed only by first-flash experiments following excessive dark adaptation. In addition, the docking site of flavodoxin on photosystem I was determined by electron microscopy in combination with image analysis. Flavodoxin binds to the cytoplasmic side of photosystem I at a distance of 7 nm from the centre of the trimer and in close contact to a ridge formed by the subunits PsaC, PsaD and PsaE.
Original languageEnglish
Pages (from-to)488-497
Number of pages10
JournalEMBO Journal
Volume15
Issue number3
Publication statusPublished - 1-Feb-1996

Keywords

  • cyanobacteria
  • electronmicroscopy
  • flavodoxin
  • photosystem I
  • spectroscopy
  • FERREDOXIN-NADP+ REDUCTASE
  • SYNECHOCYSTIS PCC-6803
  • OXIDIZED FLAVODOXIN
  • PCC-7119 FLAVODOXIN
  • ANACYSTIS-NIDULANS
  • ANABAENA PCC-7119
  • SYNECHOCOCCUS SP
  • ALGA
  • RESOLUTION
  • FLAVIN

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