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Iron deficiency induces the formation of an antenna ring around trimeric photosystem I in cyanobacteria


Although iron is the fourth most abundant element in the Earth's crust, its concentration in the aquatic ecosystems—particularly the open oceans—is sufficiently low to limit photosynthetic activity and phytoplankton growth1,2. Cyanobacteria, a major class of phytoplankton, respond to iron deficiency by expressing the ‘iron-stress-induced’ gene, isiA(ref. 3). The protein encoded by this gene has an amino-acid sequence that shows significant homology with one of the chlorophyll a-binding proteins (CP43) of photosystem II (PSII)4,5. The precise function of the CP43-like protein, here called CP43′, has not been elucidated, although there have been many suggestions3,6. Here we show that CP43′ associates with photosystem I (PSI) to form a complex that consists of a ring of 18 CP43′ molecules around a PSI trimer. This significantly increases the size of the light-harvesting system of PSI. The utilization of a PSII-like protein as an extra antenna for PSI emphasises the flexibility of cyanobacterial light-harvesting systems, and seems to be a strategy which compensates for the lowering of phycobilisome and PSI levels in response to iron deficiency.

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Figure 1: SDS–PAGE of fractions derived from sucrose density centrifugation.
Figure 2: Optical absorption and fluorescence properties of fractions derived from sucrose density centrifugation.
Figure 3: Electron micrographs and processed top views of the PSI trimer and CP43–PS1 supercomplex.
Figure 4: The PSI trimer structure and CP43 helix organization derived from X-ray crystallography10,11 overlaid onto the projection map of the CP43′–PSI complex.


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We thank A. Telfer, B. Hankamer, E. Morris, C. Büchel, J. Duncan and P. Da Fonseca for discussions; T. Bricker for supplying the His-tagged Synechocystis PCC 6803 mutant; and L. Sherman for supplying the CP43′ antibody. This work was supported by the Biotechnology and Biological Research Council.

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Correspondence to James Barber.

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Bibby, T., Nield, J. & Barber, J. Iron deficiency induces the formation of an antenna ring around trimeric photosystem I in cyanobacteria. Nature 412, 743–745 (2001).

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