Supramolecular organization and dual function of the IsiA chlorophyll-binding protein in cyanobacteria

A significant part of global primary productivity is provided by cyanobacteria, which are abundant in most marine and freshwater habitats. In many oceanographic regions, however, the concentration of iron can be so low that it limits 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'. It was recently shown that 18 IsiA proteins encircle trimeric photosystem I (PSI) under iron-deficient growth conditions. We report here that after prolonged growth of Synechocystis PCC 6803 in an iron-deficient medium, the number of bound IsiA proteins can be much higher than previously known. The largest complexes bind 12-14 units in an inner ring and 19-21 units in an outer ring around a PSI monomer. Fluorescence excitation spectra indicate an efficient light harvesting function for all PSI-bound chlorophylls. We also find that IsiA accumulates in cyanobacteria in excess of what is needed for functional light harvesting by PSI, and that a significant part of IsiA builds supercomplexes without PSI. Because the further 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 suggest that IsiA plays a surprisingly versatile role in cyanobacteria, by significantly enhancing the light harvesting ability of PSI and providing photoprotection for PSII.