Abstract
The cyanobacterium Synechocystis PCC 6803 has been subjected to growth under iron-deficient conditions. As a consequence, the isiA gene is expressed, and its product, the chlorophyll a-binding protein CP43', accumulates in the cell. Recently, we have shown for the first time that 18 copies of this photosystem II (PSII)-like chlorophyll a-binding protein forms a ring around the trimeric photosystem I (PSI) reaction center (Bibby, T. S., Nield, J., and Barber, J. (2001) Nature, 412, 743-745). Here we further characterize the biochemical and structural properties of this novel CP43'-PSI supercomplex confirming that it is a functional unit of approximately 1900 kDa where the antenna size of PSI is increased by 70% or more. Using electron microscopy and single particle analysis, we have constructed a preliminary three-dimensional model of the CP43'-PSI supercomplex and used it as a framework to incorporate higher resolution structures of PSI and CP43 recently derived from x-ray crystallography. Not only does this work emphasize the flexibility of cyanobacterial light-harvesting systems in response to the lowering of phycobilisome and PSI levels under iron-deficient conditions, but it also has implications for understanding the organization of the related chlorophyll a/b-binding Pcb proteins of oxychlorobacteria, formerly known as prochlorophytes.
Highlights
Iron is the most abundant transition metal in the crust of the earth and is an absolute requirement for photosynthetic organisms such as cyanobacteria, because it is needed for many of the redox reactions of the photosynthetic electron transport system
There have been at least four postulates. (i) CP43Ј aids the recovery of cells by acting as a chlorophyll store so that photosystem II (PSII) and photosystem I (PSI) complexes can be quickly synthesized when iron becomes readily available in the environment [12]. (ii) CP43Ј protects PSII from photo-induced damage by acting as a dissipater of excitation energy [13]. (iii) CP43Ј is a functional replacement for CP43 in PSII during iron starvation [8]. (iv) CP43Ј acts as a light-harvesting complex under iron stress conditions, mainly for PSII [5] but perhaps for PSI [16]
We have found that in addition to these well recognized responses to iron limitation, Synechocystis forms a supercomplex composed of a ring of 18 copies of the CP43Ј protein surrounding a PSI trimer
Summary
Iron is the most abundant transition metal in the crust of the earth and is an absolute requirement for photosynthetic organisms such as cyanobacteria, because it is needed for many of the redox reactions of the photosynthetic electron transport system. Cyanobacteria and other microorganisms have evolved a number of responses to cope with frequently occurring conditions of iron deficiency [4]. One such response is to express two “iron stress-induced” genes, isiA and isiB [5, 6], which are located on the same operon. (iv) CP43Ј acts as a light-harvesting complex under iron stress conditions, mainly for PSII [5] but perhaps for PSI [16]. We showed for the first time that a CP43Ј-PSI trimer supercomplex can be isolated from the cyanobacteria Synechocystis PCC 6803 when grown under iron-stressed conditions [17]. We report a more detailed description of this supercomplex and present a preliminary three-dimensional model of its structure
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