Photoprotective function of IsiA in Synechocystis PCC 6803

Due to the low solubility of iron, cyanobacteria need to have adaptation mechanisms of their photosynthetic apparatus. IsiA (iron-stress-inducible protein A) has been found as a ring of 18 subunits surrounding PSI trimers under limited stress conditions (Boekema et al., Nature 2001, Bibby et al., Nature 2001). Under prolonged iron deficiency, aggregates of many IsiA subunits are synthesized, some of them not associated with PSI (Yeremenko et al. Biochemistry 2004). We purified IsiA aggregates in the absence of PSI, and found that they contain chlorophyll a, ß-carotene and zeaxanthin. The IsiA aggregates show an extraordinary temperature dependence of the fluorescence emission yield at 687 nm, which was also observed in the membranes from which the aggregates were extracted. At room temperature, the fluorescence quantum yield of IsiA aggregates is much smaller than that of monomeric chlorophyll a in acetone. Time-resolved fluorescence experiments revealed a main lifetime for IsiA aggregates in the hundreds of pico-seconds time range. These results suggest that IsiA aggregates are directly involved in non-photochemical quenching and confirm the photoprotection ability of IsiA under stress conditions.