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This chapter reviews our knowledge on the antenna system of higher plants’ Photosystem I (PSI). Four polypeptides, denoted Lhca1, Lhca2, Lhca3 and Lhca4, are the major components of this antenna system (LHCI).

In the first part the association of the antenna with the core and the supramolecular organization of the PSI core with its Lhca-subunits are discussed. The three-dimensional (3D) structure of a PSI-LHCI supercomplex is now available and it shows that one copy of each Lhca1 to Lhca4 is bound asymmetrically to one side of the PSI core. Complementary biochemical and mutational studies have revealed that strong inter-subunit interactions maintain the association between core and antenna and lead to a very stable PSI supercomplex.

The second part describes the biochemical and spectroscopic properties of the antenna system. Under native conditions the isolation of individual antenna subunits from the PSI core is very difficult due to the stability of the PSI supercomplex. For this reason alternative approaches, such as in vitro reconstitution and reverse genetics, have furnished important contributions to the characterization of the individual polypeptides that compose the PSI antenna system.

The third part focuses on a unique spectroscopic property of Photosystem I, i.e., the presence of Chl a molecules with Qy transitions at unusual long wavelengths. These special Chls denoted “red forms” represent the most striking example of the ability of the protein scaffold to modulate the physicochemical properties of pigments. In higher plants, the “red forms” have been localized in the antenna system, in particular in Lhca4 and Lhca3. Mutational studies, moreover, allowed the identification of two specific chlorophylls, establishing excitonic interactions as responsible for the red absorption.

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