Role of Echinenone in Fluorescence Quenching in IsiA Aggregates from Cyanobacteria

We compare the carotenoid composition and the quenching properties of IsiA aggregates obtained from a Synechocystis PCC 6803 psaFJ-null mutant and from a crtO-null mutant that is unable to synthesize echinenone. Pigment analysis indicates that isolated IsiA aggregates from the psaFJ-null mutant bind, per IsiA monomer, about 16 chlorophyll a, 2.2 β-carotene, 0.9 zeaxanthin and 1 echinenone molecules, and that isolated IsiA aggregates from the crtO-null mutant bind per 16 chlorophyll a about 3.0 β-carotene and 1.1 zeaxanthin molecules. This indicates that the echinenone binding site is occupied by β-carotene or zeaxanthin in the crtO-null mutant. The isolated IsiA aggregates from this mutant and the cells from which these aggregates were obtained show the same temperature dependence of the fluorescence quantum yields as in the corresponding materials from the psaFJ-null mutant. This suggests that echinenone is not necessarily involved in the quenching mechanism and that zeaxanthin or β-carotene could also act as quenchers.