Supramolecular Organization and Dual Function of the IsiA Chlorophyll-Binding Protein in Cyanobacteria

Abstract

A significant part of global primary productivity is provided by cyanobacteria, which are abundant in most marine and freshwater habitats. In many oceanographic regions, however, the concentration of iron can be so low that it limits growth. Cyanobacteria respond to this condition by expressing a number of iron stress inducible genes, of which the isiA gene encodes a chlorophyll-binding protein known as IsiA or CP43'. It was recently shown that 18 IsiA proteins encircle trimeric photosystem I (PSI) under iron-deficient growth conditions. We report here that after prolonged growth of Synechocystis PCC 6803 in an iron-deficient medium, the number of bound IsiA proteins can be much higher than previously known. The largest complexes bind 12-14 units in an inner ring and 19-21 units in an outer ring around a PSI monomer. Fluorescence excitation spectra indicate an efficient light harvesting function for all PSI-bound chlorophylls. We also find that IsiA accumulates in cyanobacteria in excess of what is needed for functional light harvesting by PSI, and that a significant part of IsiA builds supercomplexes without PSI. Because the further decline of PSI makes photosystem II (PSII) increasingly vulnerable to photooxidation, we postulate that the surplus synthesis of IsiA shields PSII from excess light. We suggest that IsiA plays a surprisingly versatile role in cyanobacteria, by significantly enhancing the light harvesting ability of PSI and providing photoprotection for PSII.

Aggregates of the Chlorophyll-Binding Protein IsiA (CP43‘) Dissipate Energy in Cyanobacteria

“…The IEC was monitored with an on-line diode array detector (Shimadzu SPD-M10Avp). Size-exclusion chromatography was conducted with a Superdex 200 HR 10/30 column (Pharmacia) as described previously (29).…”

“…The average time by which excitations become trapped by charge separation in the reaction center was shown to be ∼40 ps (33,34), nearly 2 times longer than in the PSI core complex. On the basis of these findings, it was suggested that the IsiA bound to PSI has a light-harvesting function and that the unconnected IsiA is involved in photoprotection (29). The photoprotection function was confirmed and extended by the recent finding that IsiA is, under iron-replete growth conditions, also induced by high light and protects the organism from photooxidative stress (35).…”

Transcript Profiling Reveals New Insights into the Acclimation of the Mesophilic Fresh-Water CyanobacteriumSynechococcus elongatusPCC 7942 to Iron Starvation

“…A similar result was also obtained from Synechocystis sp. PCC 6803 under prolonged Fe limitation (Yeremenko et al, 2004).…”

An internal antisense RNA regulates expression of the photosynthesis gene isiA

“…Activation of its expression under iron limitation is mediated by the ferric uptake regulator Fur (6). IsiA also functions to dissipate excess light energy under high light and oxidative stress (7)(8)(9), and thus the precise cellular role of IsiA and the regulation of its gene expression under these conditions have remained enigmatic (6,(10)(11)(12)(13). Here, we report the identification of a unique regulatory mechanism acting at the posttranscriptional level by targeted degradation of duplexes formed between the isiA mRNA and its antisense regulator, IsrR (iron stress-repressed RNA).…”