title: The Presence of the IsiA-PSI Supercomplex Leads to Enhanced Photosystem I Electron Throughput in Iron-Starved Cells of Synechococcus sp. PCC 7002.
abstract: Photosystem I (PS I) is highly demanding of iron, requiring 12 atoms in the bound FX, FB, and FA iron-sulfur clusters and two atoms in the mobile acceptor protein ferredoxin. When grown under iron-limiting conditions, certain cyanobacteria express IsiA, a peripheral chlorophyll a antenna protein, and IsiB, a flavodoxin that substitutes for ferredoxin. The IsiA protein forms single and double rings around PS I, presumably to increase the optical cross-section so as to compensate for fewer PS I complexes. Previous studies have shown that IsiA serves as an efficient light-harvesting structure ( Andrizhievskaya , G. G. ; et al. Biochim. Biophys. Acta 2002 , 1556 , 262 - 272 ); however, few, if any, studies have been carried out to show that the increased optical cross-section leads to an enhanced rate of electron transfer through PS I. Here, we report a more rapid transient accumulation of the A1(-) phyllosemiquinone anion radical by EPR spectroscopy in dark-adapted iron-depleted cells than in iron-replete cells after a block of intense light. A derivative-shaped optical signal in the light-minus-dark difference spectrum of PS I from an electrochromic bandshift of a carotenoid located near the A1 phylloquinones is enhanced in iron-depleted wild-type cells and in an iron-depleted isiB deletion strain, which lacks flavodoxin, but is greatly diminished in an iron-depleted isiA deletion strain, which lacks IsiA and flavodoxin. These findings indicate that the transient accumulation of electrons on A1 occurs more rapidly in the IsiA/PS I supercomplex than in the PS I complex alone. Thus, the increased absorption cross-section from the IsiA proteins translates directly to an enhanced rate of electron transfer through PS I.
authors.affiliation: Department of Biochemistry and Molecular Biology and ‡Department of Chemistry, The Pennsylvania State University , University Park, Pennsylvania 16802 United States.
authors.forename: Junlei
authors.initials: J
authors.lastname: Sun
authors.affiliation: Department of Biochemistry and Molecular Biology and ‡Department of Chemistry, The Pennsylvania State University , University Park, Pennsylvania 16802 United States.
authors.forename: John H
authors.initials: JH
authors.lastname: Golbeck
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry / Q000378:metabolism*; D004579:Electron Transport; D007501:Iron / Q000378:metabolism*; D045342:Light-Harvesting Protein Complexes / Q000737:chemistry / Q000378:metabolism*; D045331:Photosystem I Protein Complex / Q000737:chemistry / Q000378:metabolism*; D046940:Synechococcus / Q000166:cytology / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; D045342:Light-Harvesting Protein Complexes; D045331:Photosystem I Protein Complex; C076726:chlorophyll A binding protein CP43, Cyanobacteria; D007501:Iron