Spectroscopic properties of PSI-IsiA supercomplexes from the cyanobacterium Synechococcus PCC 7942

Biochim Biophys Acta. 2002 Dec 2;1556(2-3):265-72. doi: 10.1016/s0005-2728(02)00371-7.


The cyanobacterium Synechococcus PCC 7942 grown under iron starvation assembles a supercomplex consisting of a trimeric Photosystem I (PSI) complex encircled by a ring of 18 CP43' or IsiA light-harvesting complexes [Nature 412 (2001) 745]. Here we present a spectroscopic characterization by temperature-dependent absorption and fluorescence spectroscopy, site-selective fluorescence spectroscopy at 5 K, and circular dichroism of isolated PSI-IsiA, PSI and IsiA complexes from this cyanobacterium grown under iron starvation. The results suggest that the IsiA ring increases the absorption cross-section of PSI by about 100%. Each IsiA subunit binds about 16-17 chlorophyll a (Chl a) molecules and serves as an efficient antenna for PSI. Each of the monomers of the trimeric PSI complex contains two red chlorophylls, which presumably give rise to one exciton-coupled dimer and at 5 K absorb and fluoresce at 703 and 713 nm, respectively. The spectral properties of these C-703 chlorophylls are not affected by the presence of the IsiA antenna ring. The spectroscopic properties of the purified IsiA complexes are similar to those of the related CP43 complex from plants, except that the characteristic narrow absorption band of CP43 at 682.5 nm is missing in IsiA.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Bacterial Proteins / chemistry*
  • Bacterial Proteins / metabolism
  • Carrier Proteins / chemistry*
  • Carrier Proteins / metabolism
  • Circular Dichroism
  • Cyanobacteria / chemistry*
  • Cyanobacteria / metabolism
  • Light-Harvesting Protein Complexes*
  • Macromolecular Substances
  • Photosynthetic Reaction Center Complex Proteins / chemistry*
  • Photosynthetic Reaction Center Complex Proteins / metabolism
  • Photosystem I Protein Complex
  • Protein Subunits / chemistry
  • Protein Subunits / metabolism
  • Spectrometry, Fluorescence


  • Bacterial Proteins
  • Carrier Proteins
  • Light-Harvesting Protein Complexes
  • Macromolecular Substances
  • Photosynthetic Reaction Center Complex Proteins
  • Photosystem I Protein Complex
  • Protein Subunits
  • chlorophyll A binding protein CP43, Cyanobacteria