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We demonstrated previously that Synechococcus sp. PCC 7942 cells growing under Fe-deficient conditions demonstrated profound alterations in photosynthesis. These changes were seen as a shift in the major absorption peak to 674 nm and the 77 K fluorescence peak to ~685 nm. These spectral shifts were found to be due to the presence of a newly synthesized CP complex, CP43', which we determined was encoded by the isiA gene (Burnap et al., 1993, Pakrasi et al., 1985; Riethman and Sherman, 1988). At the same time, virtually no phycobilisomes are produced. Similar changes in pigment composition have been found in Synechocystis and Anabaena and the isiA gene has been found in both organisms. The exact function of CP43' is not entirely understood, but it certainly functions as a photon dissipater which protects PSII from excess light under Fe-deficient conditions. We have also speculated that it may be involved in the reconstitution of normal membranes once Fe has been added back to the culture. However, this function remains unproven.

It is important to better understand the exact role of CP43' if we are to understand the implications of growth under Fe-deficient conditions. We had previously made a deletion mutant of isiA to demonstrate the importance of this gene under Fe-deficient conditions and demonstrated that there was a threshold of Fe concentration below which cells would not grow. Furthermore, we postulated that CP43' might be related to the chlorophyll a/b binding proteins seen in the prochlorophytes. This led us into our work with Prochlorothrix hollandica and this proposal was eventually proven correct (LaRoche et al., 1996). These groups demonstrated by both gene and protein sequencing that the prochlorophyte chlorophyll a/b (Pcb) proteins were not related to the chlorophyll a/b proteins of higher plants and green algae, but related directly to CP43'. van der Staay et al. (1998) presented evidence that the duplication of the isiA-like ancestor gave rise to the Pcbs and to the modern isiAs. Thus, the importance of Fe-deficiency in the overall natural history of these marine organisms must be profound. Some transcriptional analysis of the isiAB operon has been performed in salt-stressed cells of Synechocystis sp. PCC 6803 (Vinnemeier et al., 1998) and a Fur-binding site has been detected.

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