PARENT SESSION
Posters P4Aa Chlorophyll and bilin based antenna systems. Abstracts (239-271)

Structural characterization of PSI-IsiA complexes in iron-stressed cyanobacteria. Roman Kouril^*,1, Nataliya Yeremenko², Janne Ihalainen³, Sandrine D'Haene³, Niels van oosterwijk¹, Hans Matthijs², Jan Dekker³, Egbert Boekema¹, ¹ Biophysical Chemistry, Netherlands² Aquatic Microbiology, Netherlands³ Physics and Astronomy, Netherlands

ABSTRACT- In many habitats the concentration of iron may be so low that it will limit growth of cyanobacteria. They respond to this condition by producing the iron-stress-inducible protein IsiA. It was shown that 18 IsiA proteins encircle trimeric photosystem I (PSI) under short iron-deficiency. We investigated the structures of IsiA and PSI-IsiA supercomplexes under conditions of prolonged iron limitation with wild-type Synechocystis PCC 8603 by electron microscopy (EM) and image analysis. After about 20 days of culture single and double closed rings of IsiA around a monomeric PSI are present in significant numbers, besides the standard PSI trimer plus 18 IsiA copies. The double ring structures consists of 12, 13, 14 units in the inner ring and 19, 20 and 21 copies in the outer ring, respectively. The largest particles with a double IsiA ring around a monomeric PSI would contain about 660 chlorophylls. PSI particles with such large antenna sizes have not been observed before. The about 7-fold increased antenna size will thus provide compensation for the light need of the lower number of PSI centers compared to PSII under iron stress. EM and fluorescence emission spectroscopy data also show that cyanobacteria accumulate IsiA in excess of what is needed for functional light harvesting for PSI, because many double rings do not contain PSI. Whether this excess synthesis of IsiA serves to protect PSII, which is very vulnerable to photooxidation in the absence of PSI is not clear yet. Our results highlight the capacity of the IsiA protein to form flexible supramolecular structures of variable size to optimally respond to the degree of iron stress.

KEY WORDS: IsiA, iron-stress, Photosystem I, supercomplex