The chlorophyll-binding protein IsiA is inducible by high light and protects the cyanobacterium Synechocystis PCC6803 from photooxidative stress

The products of the IsiAB operon are a chlorophyll antenna protein (IsiA) and flavodoxin (IsiB), which accumulate in cyanobacteria grown under iron starvation conditions. Here we show that strong light triggers de-repression of IsiAB transcription and leads to IsiA and flavodoxin accumulation under iron replete conditions. Genetic deletion of IsiAB resulted in a photosensitive phenotype, with accumulation of reactive oxygen species and cell bleaching in high light, while the Havodoxin-deficient isiB null mutant expressing isiA was phototolerant. We conclude that IsiA protects cyanobacteria from photooxidative stress. IsiA is the first example of a chlorophyll antenna protein outside the extended LHC family that is induced transiently by high light and that fulfills a photoprotective role. (c) 2005 Published by Elsevier B.V. on behalf of the Federation of European Biochemical Societies.