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ENVIRONMENTAL STRESS AND ADAPTATION TO STRESS

Iron Deficiency in Cyanobacteria Causes Monomerization of Photosystem I Trimers and Reduces the Capacity for State Transitions and the Effective Absorption Cross Section of Photosystem I in Vivo¹

Department of Biology and The Biotron, University of Western Ontario, London, Ontario, Canada N6A 5B7 (A.G.I., M. Krol, N.P.A.H.); Department of Plant Physiology, University of Umeå, Umea S–901 87, Sweden (A.G.I., D.S., E.S., S.S., G.O.); Department of Biological Sciences, Brock University, St. Catharines, Ontario, Canada L2S 3A1 (M. Koochek, S.V., D.B.); and Department of Biology, Chungnam National University, Daejon 305–764, Korea (Y.-I.P.)

The induction of the isiA (CP43') protein in iron-stressed cyanobacteria is accompanied by the formation of a ring of 18 CP43' proteins around the photosystem I (PSI) trimer and is thought to increase the absorption cross section of PSI within the CP43'-PSI supercomplex. In contrast to these in vitro studies, our in vivo measurements failed to demonstrate any increase of the PSI absorption cross section in two strains (Synechococcus sp. PCC 7942 and Synechocystis sp. PCC 6803) of iron-stressed cells. We report that iron-stressed cells exhibited a reduced capacity for state transitions and limited dark reduction of the plastoquinone pool, which accounts for the increase in PSII-related 685 nm chlorophyll fluorescence under iron deficiency. This was accompanied by lower abundance of the NADP-dehydrogenase complex and the PSI-associated subunit PsaL, as well as a reduced amount of phosphatidylglycerol. Nondenaturating polyacrylamide gel electrophoresis separation of the chlorophyll-protein complexes indicated that the monomeric form of PSI is favored over the trimeric form of PSI under iron stress. Thus, we demonstrate that the induction of CP43' does not increase the PSI functional absorption cross section of whole cells in vivo, but rather, induces monomerization of PSI trimers and reduces the capacity for state transitions. We discuss the role of CP43' as an effective energy quencher to photoprotect PSII and PSI under unfavorable environmental conditions in cyanobacteria in vivo.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Norman P.A. Huner (nhuner{at}uwo.ca).

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.106.082339.

^* Corresponding author; e-mail nhuner{at}uwo.ca; fax 519–661–3935.

Received April 18, 2006; returned for revision June 8, 2006; accepted June 8, 2006.

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