Three-dimensional model and characterization of the iron stress-induced CP43 '-photosystem I supercomplex isolated from the cyanobacterium Synechocystis PCC 6803

The cyanobacterium Synechocystis PCC 6803 has been subjected to growth unde r iron-deficient conditions. As a consequence, the isiA gene is expressed, and its product, the chlorophyll a-binding protein CP43', accumulates in th e cell. Recently, we have shown for the first time that 18 copies of this p hotosystem 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 supercomp lex 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 micros copy 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 derive d from x-ray crystallography. Not only does this work emphasize the flexibi lity of cyanobacterial light-harvesting systems in response to the lowering of phycobilisome and PSI levels under iron-deficient conditions, but it al so has implications for understanding the organization of the related chlor ophyll a/b-binding Pcb proteins of oxychlorobacteria, formerly known as pro chlorophytes.