Supercomplexes of IsiA and Photosystem I in a mutant lacking subunit PsaL

Functional flexibility of light harvesting in cyanobacteria

Nataliya Yeremenko 1, Roman Kouril 2, Janne A. Ihalainen 3, Sandrine D¿Haene 3, Martin Hagemann 4, Egbert J. Boekema 2, Hans C.P. Matthijs 1 and Jan P. Dekker 3
1 Universiteit van Amsterdam, Amsterdam, The Netherlands;
2 University of Groningen, Groningen, The Netherlands;
3 Vrije Universiteit, Amsterdam, The Netherlands;
4 Universität Rostock, Rostock, Germany

Regulation of light harvesting is a matter of high importance in photosynthetic organisms. High plants and algae involve mechanism of state transitions which is regulated by the redox state of the plastoquinone pool and leads to the reversible redistribution of excitation energy between the two photosystems through a reorganization of the antennae and to an overall increase in photosynthetic quantum yield by decreasing the delivery of excess excitation energy to PSII while increasing delivery of excitation energy to PSI (or vice-versa) [1]. Cyanobacteria balance energy input and consumption differently. Under iron-deficient conditions, the content of PSI decreases substantially more than that of PSII [2]. Cyanobacteria respond to this condition by accumulation of IsiA protein which may occurs in two different types of complexes (associated and not associated with PSI) and correspondingly serve different functions [3]. The PSI-IsiA supercomplexes show increasingly larger amounts of bound IsiA proteins with increasing iron starvation. The largest particles consist of double IsiA ring around monomeric PSI and contain about 560 chlorophylls, which together with 96 chlorophylls of the PSI core complex would give a total number of 656 chlorophylls. PSI particles with such large antenna sizes have not been observed before. Fluorescence excitation spectra indicate an efficient light-harvesting function for all PSI-bound chlorophylls. With continious iron starvation the further decline of PSI makes PSII increasingly vulnerable to photooxidation and the surplus synthesis of IsiA not associated with PSI shields PSII from excess light.