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Accumulation and light-harvesting function of IsiA in cyanobacterial cells with monomeric and trimeric Photosystem I
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  • Petar Lambrev,
  • Parveen Akhtar,
  • Fanny Balog-Vig,
  • Soujanya Kuntam,
  • Szilvia Z. Tóth
Petar Lambrev
Szegedi Biologiai Kutatokozpont Novenybiologiai Intezet

Corresponding Author:[email protected]

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Parveen Akhtar
Szegedi Biologiai Kutatokozpont Novenybiologiai Intezet
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Fanny Balog-Vig
Szegedi Biologiai Kutatokozpont Novenybiologiai Intezet
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Soujanya Kuntam
Szegedi Biologiai Kutatokozpont Novenybiologiai Intezet
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Szilvia Z. Tóth
Szegedi Biologiai Kutatokozpont Novenybiologiai Intezet
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Abstract

Iron deficiency is a common challenge faced by cyanobacteria in oceanic environments and the acclimation to iron deficiency is crucial for their survival. In response to iron deficiency, many cyanobacterial species induce the production of a pigment-protein complex called IsiA. IsiA proteins associate with photosystem I (PSI) and can function as light-harvesting antennas or dissipate excess energy. They may also serve as Chl storage during iron limitation and aid in the recovery of the photosynthetic apparatus. In this study we examined the functional role of IsiA in cells of Synechocystis sp. PCC 6803 grown under iron-limitation conditions by measuring the cellular IsiA content and its capability to transfer energy to PSI. We specifically test the effect of the oligomeric state of PSI by comparing wild-type (WT) Synechocystis sp. PCC 6803 to mutants lacking specific subunits of PSI, namely PsaL/PsaI (Δ psaL mutant) and PsaF/PsaJ (ΔFIJL mutant). Time-resolved fluorescence spectroscopy revealed that IsiA formed functional PSI 3-IsiA 18 supercomplexes, wherein IsiA effectively transfers energy to PSI on a timescale of 10 ps at room temperature, confirming the primary role of IsiA as an accessory light-harvesting antenna to PSI. However, a significant fraction (40%) remained unconnected to PSI in the form of IsiA aggregates, supporting the notion of a dual functional role of IsiA. The absence of PsaL and PsaF subunits led to reduced accumulation of IsiA under iron deficiency with PSI monomers only binding 3-4 IsiA complexes on average. Taken together, the results show that IsiA is capable of transferring energy to trimeric and to monomeric PSI but to varying degrees and that the acclimatory production of IsiA under iron stress is controlled by its ability to perform its light-harvesting function.