The Presence of the IsiA-PSI Supercomplex Leads to Enhanced Photosystem i Electron Throughput in Iron-Starved Cells of Synechococcus sp. PCC 7002

Photosystem I (PS I) is highly demanding of iron, requiring 12 atoms in the bound F_X, F_B, and F_A 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.