Reevaluating the mechanism of excitation energy regulation in iron-starved cyanobacteria

This paper presents spectroscopic investigations of IsiA, a chlorophyll a-binding membrane protein produced by cyanobacteria grown in iron-deficient environments. IsiA, if associated with photosystem I, supports photosystem I in light harvesting by efficiently transferring excitation energy. However, if separated from photosystem I, IsiA exhibits considerable excitation quenching observed as a substantial reduction of protein-bound chlorophyll a fluorescence lifetime. Previous spectroscopic studies suggested that carotenoids are involved in excitation energy dissipation and in addition play a second role in this antenna complex by supporting chlorophyll a in light harvesting by absorbing in the spectral range inaccessible for chlorophyll a and transferring excitation to chlorophylls. However, this investigation does not support these proposed roles of carotenoids in this light harvesting protein. This study shows that carotenoids do not transfer excitation energy to chlorophyll a. In addition, our investigations do not support the hypothesis that carotenoids are quenchers of the excited state of chlorophyll a in this protein complex. We propose that quenching of chlorophyll a fluorescence in IsiA is maintained by pigment-protein interaction via electron transfer from an excited chlorophyll a to a cysteine residue, an excitation quenching mechanism that was recently proposed to regulate the light harvesting capabilities of the bacteriochlorophyll a-containing Fenna-Mathews-Olson protein from green sulfur bacteria.