title: Light-induced energy dissipation in iron-starved cyanobacteria: roles of OCP and IsiA proteins.
abstract: In response to iron deficiency, cyanobacteria synthesize the iron stress-induced chlorophyll binding protein IsiA. This protein protects cyanobacterial cells against iron stress. It has been proposed that the protective role of IsiA is related to a blue light-induced nonphotochemical fluorescence quenching (NPQ) mechanism. In iron-replete cyanobacterial cell cultures, strong blue light is known to induce a mechanism that dissipates excess absorbed energy in the phycobilisome, the extramembranal antenna of cyanobacteria. In this photoprotective mechanism, the soluble Orange Carotenoid Protein (OCP) plays an essential role. Here, we demonstrate that in iron-starved cells, blue light is unable to quench fluorescence in the absence of the phycobilisomes or the OCP. By contrast, the absence of IsiA does not affect the induction of fluorescence quenching or its recovery. We conclude that in cyanobacteria grown under iron starvation conditions, the blue light-induced nonphotochemical quenching involves the phycobilisome OCP-related energy dissipation mechanism and not IsiA. IsiA, however, does seem to protect the cells from the stress generated by iron starvation, initially by increasing the size of the photosystem I antenna. Subsequently, the IsiA converts the excess energy absorbed by the phycobilisomes into heat through a mechanism different from the dynamic and reversible light-induced NPQ processes.
authors.affiliation: Unité de Recherche Associée 2096, Centre National de la Recherche Scientifique, Service de Bioénergétique, Commissariat à l'Energie Atomique Saclay, 91191 Gif sur Yvette, France.
authors.forename: Adjélé
authors.initials: A
authors.lastname: Wilson
authors.forename: Clémence
authors.initials: C
authors.lastname: Boulay
authors.forename: Annegret
authors.initials: A
authors.lastname: Wilde
authors.forename: Cheryl A
authors.initials: CA
authors.lastname: Kerfeld
authors.forename: Diana
authors.initials: D
authors.lastname: Kirilovsky
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D002734:Chlorophyll / Q000378:metabolism; D004734:Energy Metabolism; D000090463:Iron Deficiencies; D008027:Light; D045342:Light-Harvesting Protein Complexes / Q000235:genetics / Q000378:metabolism*; D045332:Photosystem II Protein Complex / Q000235:genetics / Q000378:metabolism; D045524:Phycobilisomes / Q000378:metabolism; D013050:Spectrometry, Fluorescence; D013217:Starvation; D046939:Synechocystis / Q000166:cytology / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; D045342:Light-Harvesting Protein Complexes; D045332:Photosystem II Protein Complex; D045524:Phycobilisomes; C109473:orange carotenoid protein, Synechocystis; C070970:photosystem II, chlorophyll-binding protein, CP-47; D002734:Chlorophyll; C076726:chlorophyll A binding protein CP43, Cyanobacteria