Abstract
Iron deficiency and other stress conditions strongly impact photosynthetic apparatus in photosynthetic organisms. Two novel chlorophyll (Chl)-containing supercomplexes (F4 and F5) in addition to the photosystem (PS) I trimers (F3) were observed by sucrose gradient ultracentrifugation in Synechocystis PCC 6803 under extensive iron starvation. 77K fluorescence and Western blot analyses of these supercomplexes revealed that they all contained IsiA. The F4 was identified as an IsiA-PSI-PSII supercomplex, while the F5 was assigned as an IsiA-PSI supercomplex. Deletion of isiA resulted in diminishing the PSI trimers (including the PSI trimers in iron-replete cells) and the two novel PSI supercomplexes (F4 and F5), and a significant reduction in the saturated whole-chain electron transport rate. However, the maximum PSII activities remained at levels similar to those of the wild type under various light conditions. The isiA- mutant was defective in state transition and sensitive to high light. The sensitivity of the isiA- mutant to high light was correlated with a higher level of membrane peroxidation. These results demonstrated that IsiA is required for the formation of PSI trimers and other higher complexes, and that IsiA is critical for efficient state transition.
Highlights
The photosynthetic apparatus of cyanobacteria, containing 22– 23 iron atoms [1], is highly vulnerable to iron deficiency [1,2]
Iron starvation results in various structural and functional changes within the cyanobacterial cells [3,4,5,6,7,8,9]; among these changes is the appearance of a chlorophyll (Chl)-protein complex associated with the isiA gene product IsiA, or CP43’ [10,11,12,13]
It has been shown that IsiA is important for the survival of cyanobacterial cells in highintensity light (HL) [18,33,34,35]
Summary
The photosynthetic apparatus of cyanobacteria, containing 22– 23 iron atoms [1], is highly vulnerable to iron deficiency [1,2]. While the protein is most strongly induced by iron starvation and becomes the major Chl-binding protein within the cells under this condition [10,14,15], it has been reported to be inducible to some extent (at least transcriptionally) by other environmental stresses, including salt stress, heat stress, oxidative stress, and high or limited light conditions [16,17,18,19,20,21,22,23] These studies, did not provide evidence that isiA gene transcription under non-iron-limiting conditions is accompanied by protein synthesis and membrane integration of IsiA [24]. No biochemical analyses of IsiA or its associated protein complexes were performed in these studies
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