Cyanobacteria are abundant throughout most of the world's water bodies and contribute significantly to global primary productivity through oxygenic photosynthesis. This reaction is catalysed by two membrane-bound protein complexes, photosystem I (PSI) and photosystem II (PSII), which both contain chlorophyll-binding subunits functioning as an internal antenna. In addition, phycobilisomes act as peripheral antenna systems, but no additional light-harvesting systems have been found under normal growth conditions. Iron deficiency, which is often the limiting factor for cyanobacterial growth in aquatic ecosystems, leads to the induction of additional proteins such as IsiA (ref. 3). Although IsiA has been implicated in chlorophyll storage, energy absorption and protection against excessive light, its precise molecular function and association to other proteins is unknown. Here we report the purification of a specific PSI-IsiA supercomplex, which is abundant under conditions of iron limitation. Electron microscopy shows that this supercomplex consists of trimeric PSI surrounded by a closed ring of 18 IsiA proteins binding around 180 chlorophyll molecules. We provide a structural characterization of an additional chlorophyll-containing, membrane-integral antenna in a cyanobacterial photosystem.

In this review we give an overview on the adaptational responses of photosystem (PS) II and PSI in cyanobacteria to iron starvation, mainly summarizing our results with the mesophilic Synechococcus elongatus PCC 7942. We also discuss this process with respect to the strong interrelationship between iron limitation and oxidative stress that exists in cyanobacteria as oxygenic photosynthetic organisms. The adaptation of the multiprotein complexes PSII and PSI to iron starvation is a sequential process, which is characterized by the enhanced expression of two major iron-regulated proteins, IdiA (iron deficiency induced protein A) and IsiA (iron stress induced protein A). Our results suggest that IdiA protects the acceptor side of PSII against oxidative stress under conditions of mild iron limitation in a currently unclear way, whereas prolonged iron deficiency leads to the synthesis of a chlorophyll a antenna around PSI-trimers consisting of IsiA molecules. The physiological consequences of these alterations under prolonged iron starvation, as shown by acridine yellow fluorescence measurements, are a reduction of linear electron transport activity through PSII and an increase of cyclic electron flow around PSI as well as an increase in respiratory activity. IdiA and IsiA expression are mediated by two distinct helix-turn-helix transcriptional regulators of the Crp/Fnr family. IdiB positively regulates expression of idiA under iron starvation, and Fur represses transcription of isiA under iron-sufficient conditions. Although both transcriptional regulators seem to operate independently of each other, our results indicate that a cross-talk between the signal transduction pathways exists. Moreover, IdiA as well as IsiA expression are affected by hydrogen peroxide. We suggest that due to the interdependence of iron limitation and the formation of reactive oxygen species, peroxide stress might be the superior trigger that leads to expression of these proteins under iron starvation. The modifications of PSII and PSI under iron starvation influence the redox state of redox-sensitive components of the electron transport chain, and thus the activity of metabolic pathways being regulated in dependence of the redox state of these components.

In the mesophilic cyanobacterium Synechococcus elongatus PCC 7942, iron starvation induces the expression of a number of proteins, including IdiA and IsiA. Whereas IdiA protects photosystem (PS) II under mild iron limitation against oxidative stress in a yet unknown way, prolonged iron starvation leads to the formation of the PS I-IsiA supercomplex. Transcription of idiA is positively regulated by IdiB under iron starvation, and Fur represses transcription of isiAB under iron-sufficient growth conditions. In this report, data are presented suggesting a strong interrelationship between iron homeostasis and oxidative stress in S. elongatus PCC 7942, and showing that transcription of major iron-regulated genes, such as isiA, isiAB, idiA, idiB, mapA, and irpA, is induced by oxidative stress within a few minutes by treatment of cells with hydrogen peroxide or methylviologen. The overall results suggest that isiA/isiAB as well as idiB transcription in response to oxidative stress might be controlled by a transcriptional repressor possibly of the PerR-type. This fact also explains the observed cross-talk between IdiB- and Fur-mediated transcriptional regulation of gene expression and for the role of H(2)O(2) as a superior trigger coordinating expression of iron-regulated genes under iron starvation and oxidative stress. Measuring 77 K chlorophyll a fluorescence, it is shown that hydrogen peroxide treatment causes a transient short-term modification of PS II and PS I most likely leading to increased cyclic electron transport around PS I. In this context, the intriguing observation was made that idiB is transcribed as part of an operon together with a gene encoding a potential [2Fe-2S]-protein. This protein has similarity to [Fe-S]-proteins involved in the electron transport activity of the NDH I complex in eubacteria. Since the NDH I complex is involved in cyclic electron transport activity around PS I in cyanobacteria and both adaptation to iron starvation and adaptation to oxidative stress lead to an enhanced cyclic electron transport activity around PS I, this potential [Fe-S]-protein might participate in the overall adaptational response to iron starvation and/or oxidative stress in Synechococcus.

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SummaryThe production of biodegradable polymers in transgenic plants in order to replace petrochemical compounds is an important challenge for plant biotechnology. Polyaspartate, a biodegradable substitute for polycarboxylates, is the backbone of the cyanobacterial storage material cyanophycin. Cyanophycin, a copolymer of L -aspartic acid and L -arginine, is produced via non-ribosomal polypeptide biosynthesis by the enzyme cyanophycin synthetase. A gene from Thermosynechococcus elongatus BP-1 encoding cyanophycin synthetase has been expressed constitutively in tobacco and potato. The presence of the transgene-encoded messenger RNA (mRNA) correlated with changes in leaf morphology and decelerated growth. Such transgenic plants were found to produce up to 1.1% dry weight of a polymer with cyanophycin-like properties. Aggregated material, able to bind a specific cyanophycin antibody, was detected in the cytoplasm and the nucleus of the transgenic plants.