The antenna complexes of Photosystem I of Chlamydomonas reinhardtii
Roberta Croce1, Milena Mozzo1, Manuela Mantelli3, Francesca Passarini1,3, Stefano Caffarri2, and Roberto Bassi3
1) Department of Biophysical Chemistry, University of Groningen, The Netherlands
2) Universite Aix Marseille, France
3) Dipartimento Scientifico e Tecnologico, Università di Verona, Italy
The outer antenna system of Chlamydomonas reinhardtii Photosystem I is composed of nine gene products, but due to difficulty in purification their individual properties are not known. We have investigated the functional properties of the nine Lhca antennas of Chlamydomonas, upon expression of the apoproteins in bacteria and refolding in vitro of the pigment-protein complexes. It is shown that all Lhca complexes have a red-shifted fluorescence emission as compared to the antenna complexes of Photosystem II, similar to Lhca from higher plants, but less red-shifted. Three complexes, namely Lhca2, Lhca4 and Lhca9, exhibit emission maxima above 707 nm and all carry an asparagine as ligand for Chl 603. The comparison of the protein sequences and the biochemical/spectroscopic properties of the refolded Chlamydomonas complexes with those of the well-characterized A. thaliana Lhcas shows that all the Chlamydomonas complexes have a chromophore organization similar to that of A. thaliana antennas, particularly to Lhca2, despite low sequence identity. All the major biochemical and spectroscopic properties of the Lhca complexes have been conserved through the evolution, including those involved in "red forms" absorption. The different properties of the individual Lhca complexes can be functional to adapt the architecture of the PSI-LHCI supercomplex to different environmental conditions.
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