PARENT SESSION

Symposium S8B Supermolecular organization of the photosynthetic apparatus
Friday September 3rd, 2004 8:30 AM-10:30 AM Room 210A
Chair: Jan Dekker
Co-Chair: André Verméglio

Structural characterization of PSI-ISIA supercomplexes in iron-stressed cyanobacteria lacking the PsaL subunit. Roman Kouril^*,1, Nataliya Yeremenko², Sandrine D'Haene³, Wilko Keegstra¹, Hans Matthijs², Jan Dekker³, Egbert Boekema¹, ¹ Biophysical Chemistry, Groningen, The Netherlands² Aquatic Microbiology, Amsterdam, The Netherlands³ Division of Physics and Astronomy, Amsterdam, The Netherlands

ABSTRACT- Growing of cyanobacteria under conditions of iron deficiency leads to an overall reduction of the PSI level compared to PSII, the degradation of phycobilisomes and expression of a new transmembrane light-harvesting protein - IsiA (Iron Stress Inducible protein). The role of the IsiA protein is to increase an absorption cross-section of the remaining PSI and thus compensate its imbalance to PSII. Recently, it was found that IsiA can form flexible rings around either a trimer or a monomer of PSI according to the degree of iron starvation. Studies of a mutant lacking the peripheral PsaF and PsaJ subunits indicated that these subunits are not obligatory for PSI-IsiA supercomplex formation. The involvement of PsaL, another peripheral PSI subunit, in binding of IsiA to PSI is not known yet. We focused on a structural characterization of PSI-IsiA complexes from an iron-stressed Synechocystis sp. PCC 6803 PsaL-deficient strain using a transmission electron microscopy and image analysis. After short-term iron stress (6 days) variable PSI-IsiA supercomplexes were resolved. Partial double rings of IsiA containing 15-17 units attached to the PSI monomer from the side of the PsaF and PsaJ subunits were found. In these supercomplexes, the inner half-ring appears to have a constant number of 7 copies, whereas the outer ring consists of 8-10 copies. This suggests that the mutant PSI monomer could be an inferior template for the formation of complete IsiA rings than the wild-type. On the other hand, such rings may only form under conditions of excess of IsiA. Analysis of wild type cyanobacteria after long-term iron deficiency with high levels of IsiA showed that the PSI monomer can be surrounded by complete rings with variable copies of IsiA. In the largest rings with 14+21 copies the monomer has a more tight binding at the side of the PsaF/J subunits rather than from the side of PsaL, which could indicate a minor role for the PsaL subunit in the formation of IsiA rings around PSI. We suppose that the formation of a complete IsiA rings around a PSI monomer could depend on the extend of iron stress. Further investigations of mutant complexes after long iron stress are in progress.

KEY WORDS: isi a protein, photosystem i, iron stress