title: Aggregates of the chlorophyll-binding protein IsiA (CP43') dissipate energy in cyanobacteria.
abstract: In many natural habitats, growth of cyanobacteria may be limited by a low concentration of iron. 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'. IsiA monomers assemble into ring-shaped polymers that encircle trimeric or monomeric photosystem I (PSI), or are present in supercomplexes without PSI, in particular upon prolonged iron starvation. In this report, we present steady-state and time-resolved fluorescence measurements of isolated IsiA aggregates that have been purified from an iron-starved psaFJ-minus mutant of Synechocystis PCC 6803. We show that these aggregates have a fluorescence quantum yield of approximately 2% compared to that of chlorophyll a in acetone, and that the dominating fluorescence lifetimes are 66 and 210 ps, more than 1 order of magnitude shorter than that of free chlorophyll a. Comparison of the temperature dependence of the fluorescence yields and spectra of the isolated aggregates and of the cells from which they were obtained suggests that these aggregates occur naturally in the iron-starved cells. We suggest that IsiA aggregates protect cyanobacterial cells against the deleterious effects of light.
authors.affiliation: Division of Physics and Astronomy, Faculty of Sciences, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands.
authors.forename: Janne A
authors.initials: JA
authors.lastname: Ihalainen
authors.forename: Sandrine
authors.initials: S
authors.lastname: D'Haene
authors.forename: Nataliya
authors.initials: N
authors.lastname: Yeremenko
authors.forename: Henny
authors.initials: H
authors.lastname: van Roon
authors.forename: Ana A
authors.initials: AA
authors.lastname: Arteni
authors.forename: Egbert J
authors.initials: EJ
authors.lastname: Boekema
authors.forename: Rienk
authors.initials: R
authors.lastname: van Grondelle
authors.forename: Hans C P
authors.initials: HC
authors.lastname: Matthijs
authors.forename: Jan P
authors.initials: JP
authors.lastname: Dekker
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry*; D002734:Chlorophyll / Q000737:chemistry; D000077194:Chlorophyll A; D000458:Cyanobacteria / Q000737:chemistry*; D019281:Dimerization; D004735:Energy Transfer; D006207:Half-Life; D007501:Iron / Q000378:metabolism; D008027:Light; D045342:Light-Harvesting Protein Complexes / Q000737:chemistry*; D013050:Spectrometry, Fluorescence
chemical_list: D001426:Bacterial Proteins; D045342:Light-Harvesting Protein Complexes; D002734:Chlorophyll; C076726:chlorophyll A binding protein CP43, Cyanobacteria; D007501:Iron; D000077194:Chlorophyll A