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A giant chlorophyll–protein complex induced by iron deficiency in cyanobacteria


Cyanobacteria are abundant throughout most of the world's water bodies and contribute significantly to global primary productivity through oxygenic photosynthesis. This reaction is catalysed by two membrane-bound protein complexes, photosystem I (PSI) and photosystem II (PSII), which both contain chlorophyll-binding subunits functioning as an internal antenna1. In addition, phycobilisomes act as peripheral antenna systems, but no additional light-harvesting systems have been found under normal growth conditions. Iron deficiency, which is often the limiting factor for cyanobacterial growth in aquatic ecosystems2, leads to the induction of additional proteins such as IsiA (ref. 3). Although IsiA has been implicated in chlorophyll storage, energy absorption and protection against excessive light, its precise molecular function and association to other proteins is unknown. Here we report the purification of a specific PSI–IsiA supercomplex, which is abundant under conditions of iron limitation. Electron microscopy shows that this supercomplex consists of trimeric PSI surrounded by a closed ring of 18 IsiA proteins binding around 180 chlorophyll molecules. We provide a structural characterization of an additional chlorophyll-containing, membrane-integral antenna in a cyanobacterial photosystem.

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Figure 1: Purification and subunit analysis of the PSI–IsiA complex.
Figure 2: Light-saturation curves of the absorbance changes of PSI (circles and dashed line) and the PSI–IsiA complex (squares and solid line).
Figure 3: Electron microscopy of PSI–IsiA complexes and their interpretation.


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We thank A. Kühl and F. Pfeil for technical assistance, and our colleagues M. Rögner and A. Trebst for discussions. Financial support of the Deutsche Forschungsgemeinschaft (J.K. and E.K.P.), Nederlandse Organisatie voor Wetenschappelijk Onderzoek (E.J.B.) and the Fonds der chemischen Industrie (J.K.) is gratefully acknowledged.

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Correspondence to E. K. Pistorius or J. Kruip.

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Boekema, E., Hifney, A., Yakushevska, A. et al. A giant chlorophyll–protein complex induced by iron deficiency in cyanobacteria. Nature 412, 745–748 (2001).

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