Part 2 - Introduction (contd.)

Under iron stress, the isiA gene is transcribed into two messages, a monocistronic message containing only isiA and a dicistronic message that also contains the isiB gene (10). Although the discovery of the CP43-like iron stress-induced protein was made some time ago (11), its precise function has not been elucidated. There have been at least four postulates. (i) CP43' aids the recovery of cells by acting as a chlorophyll store so that PSII and photosystem I (PSI) complexes can be quickly synthesized when iron becomes readily available in the environment (12). (ii) CP43' protects PSII from photo-induced damage by acting as a dissipater of excitation energy (13). (iii) CP43' is a functional replacement for CP43 in PSII during iron starvation (8). (iv) CP43' acts as a light-harvesting complex under iron stress conditions, mainly for PSII (5) but perhaps also for PSI (16).
      Recently we showed for the first time that a CP43'-PSI trimer supercomplex can be isolated from the cyanobacteria Synechocystis PCC 6803 when grown under iron-stressed conditions (17). Here we report a more detailed description of this supercomplex and present a preliminary three-dimensional model of its structure.