title: Fluorescence quenching of IsiA in early stage of iron deficiency and at cryogenic temperatures.
abstract: Cyanobacteria respond to iron deficiency during growth by expressing the isiA gene, which produces a chlorophyll-carotenoid protein complex known as IsiA or CP43'. Long-term iron deficiency results in the formation of large IsiA aggregates, some of which associate with photosystem I (PSI) while others are not connected to a photosystem. The fluorescence at room temperature of these unconnected aggregates is strongly quenched, which points to a photoprotective function. In this study, we report time-resolved fluorescence measurements of IsiA aggregates at low temperatures. The average fluorescence lifetimes are estimated to be about 600 ps at 5 K and 150 ps at 80 K. Both lifetimes are much shorter than that of the monomeric complex CP47 at 77 K. We conclude that IsiA aggregates quench fluorescence to a significant extent at cryogenic temperatures. We show by low-temperature fluorescence spectroscopy that unconnected IsiA is present already after two days of growth in an iron-deficient medium, when PSI and PSII are still present in significant amounts and that under these conditions the fluorescence quenching is similar to that after 18 days, when PSI is almost completely absent. We conclude that unconnected IsiA provides photoprotection in all stages of iron deficiency.
authors.affiliation: Division of Physics and Astronomy, Faculty of Sciences, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands.
authors.forename: Chantal D
authors.initials: CD
authors.lastname: van der Weij-de Wit
authors.forename: Janne A
authors.initials: JA
authors.lastname: Ihalainen
authors.forename: Edith
authors.initials: E
authors.lastname: van de Vijver
authors.forename: Sandrine
authors.initials: S
authors.lastname: D'Haene
authors.forename: Hans C P
authors.initials: HC
authors.lastname: Matthijs
authors.forename: Rienk
authors.initials: R
authors.lastname: van Grondelle
authors.forename: Jan P
authors.initials: JP
authors.lastname: Dekker
mesh_terms: D001426:Bacterial Proteins / Q000378:metabolism*; D003080:Cold Temperature; D005453:Fluorescence; D000090463:Iron Deficiencies; D045342:Light-Harvesting Protein Complexes / Q000378:metabolism*; D045331:Photosystem I Protein Complex / Q000378:metabolism; D013050:Spectrometry, Fluorescence; D046939:Synechocystis / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D045342:Light-Harvesting Protein Complexes; D045331:Photosystem I Protein Complex; C076726:chlorophyll A binding protein CP43, Cyanobacteria