PARENT SESSION
Posters P1B Photo-oxidative stress, photoinhibition. Abstracts (394-443)

Accumulation of large aggregates of chlorophyll-binding protein IsiA that are not associated with Photosystem I in a psaFJ- mutant of the cyanobacterium Synechocystis sp. PCC 6803. Nataliya Yeremenko^*,1, Roman Kouil², Janne Ihalainen³, Sandrine D’Haene³, Martin Hagemann⁴, Egbert Boekema², Jan Dekker³, Hans Matthijs¹, ¹ Aquatic Microbiology, Amsterdam, the Netherlands² Department of Biophysical Chemistry, Groningen, the Netherlands³ Division of Physics and Astronomy, Amsterdam, the Netherlands⁴ Pflanzenphysiologie, Rostock, Germany

ABSTRACT- A significant part of global primary productivity is provided by cyanobacteria, which are abundant in most marine- and fresh-water habitats. In many oceanographic regions, however, the concentration of iron may be so low that it will limit growth. Cyanobacteria respond to this condition by expressing a number of iron-stress-inducible genes, of which the isiA gene encodes a chlorophyll-binding protein known as IsiA or CP43^,. In the absence of iron, a mutant of the cyanobacterium Synechocystis sp. PCC 6803 without the photosystem I (PSI) subunits PsaF and PsaJ, shows a much stronger decrease of the PSI content than the wild-type. After prolonged growth under iron-free conditions PSI disappears nearly completely. The iron-starvation inducible protein IsiA becomes the by far most abundant chlorophyll-protein complex in the mutant. A partial purification and a characterization by mass spectrometry, spectroscopy and electron microscopy has revealed the presence of several types of IsiA aggregates, of which a ^,doughnut^,-like complex not associated with PSI is the most conspicuous. This ring-shaped complex is smaller than the normal ring of 18 IsiA proteins or the 17-mer homologue that was recently observed in the psaFJ- mutant, but large enough to encircle a monomeric PSI complex. These results suggest that the combination of oxidative stress and iron limitation that is exerted on PSI in the mutant results in the accumulation of IsiA in a form that does not contribute to light harvesting for PSI. Because the decline of PSI makes photosystem II (PSII) increasingly vulnerable to photooxidation, we postulate that the surplus synthesis of IsiA shields PSII from excess light. We conclude that IsiA plays a surprisingly versatile role in cyanobacteria, by significantly increasing the light harvesting ability of PSI and providing light shielding for PSII to prevent oxidative stress.

KEY WORDS: IsiA, iron limitation, oxidative stress, Synechocystis sp. PCC 6803