Functional analysis of the iron-stress induced CP 43? polypeptide of PS II in the cyanobacterium Synechococcus sp. PCC 7942

Abstract

Under conditions of iron-stress, the Photosystem II associated chlorophyll a protein complex designated CP 43', which is encoded by the isiA gene, becomes the major pigment-protein complex in Synechococcus sp. PCC 7942. The isiB gene, which is located immediately downstream of isiA, encodes the protein flavodoxin, which can functionally replace ferredoxin under conditions of iron stress. We have constructed two cyanobacterial insertion mutants which are lacking (i) the CP 43' apoprotein (designated isiA (-)) and (ii) flavodoxin (designated isiB (-)). The function of CP 43' was studied by comparing the cell characteristics, PS II functional absorption cross-sections and Chl a fluorescence parameters from the wild-type, isiA (-) and isiB (-) strains grown under iron-stressed conditions. In all strains grown under iron deprivation, the cell number doubling time was maintained despite marked changes in pigment composition and other cell characteristics. This indicates that iron-starved cells remained viable and that their altered phenotype suggests an adequate acclimation to low iron even in absence of CP 43' and/or flavodoxin. Under both iron conditions, no differences were detected between the three strains in the functional absorption crossection of PS II determined from single turnover flash saturation curves of Chl a fluorescence. This demonstrates that CP 43' is not part of the functional light-harvesting antenna for PS II. In the wild-type and the isiB (-) strain grown under iron-deficient conditions, CP 43' was present in the thylakoid membrane as an uncoupled Chl-protein complex. This was indicated by (1) an increase of the yield of prompt Chl a fluorescence (Fo) and (2) the persistence after PS II trap closure of a fast fluorescence decay component showing a maximum at 685 nm.

Functional Replacement of Ferredoxin by a Cyanobacterial Flavodoxin in Tobacco Confers Broad-Range Stress Tolerance

“…Cyanobacterial Fld has been shown to efficiently replace Fd in NADP þ photoreduction, nitrogen fixation, sulfite reduction, and the phosphoroclastic reaction (Falk et al, 1995). Unlike FNR and Fd, which are widely distributed among plastids, mitochondria, and bacteria, Flds are restricted to prokaryotes and some eukaryotic algae (Erdner et al, 1999).…”

“…The various Fd acceptors are organized in a hierarchical manner, attaining their maximal activities at different electronic pressures (Backhausen et al, 2000). Most of these Fd activities are also found in eukaryotic algae and in cyanobacteria, the closest living relatives of the ancient endosymbiont that gave origin to chloroplasts (Falk et al, 1995).…”

“…Photosynthetic microorganisms have adapted to the undesirable consequences of Fd decline under stress by inducing the expression of isofunctional electron carriers termed flavodoxins (Flds), small soluble proteins (;19 kD) containing one molecule of noncovalently bound flavin mononucleotide as a prosthetic group (Falk et al, 1995). Cyanobacterial Fld has been shown to efficiently replace Fd in NADP þ photoreduction, nitrogen fixation, sulfite reduction, and the phosphoroclastic reaction (Falk et al, 1995).…”

The role of the orange carotenoid protein and PsbU protein in photoprotection in cyanobacteria

“…The origin of this increase is related to the fact that the content of PSI which contains 4 Fe-S centers, decreased faster than that of PSII (Guikema and Sherman, 1983b;Sandmann, 1985;Falk et al, 1995). Another possible reason for this can be that the increase of the PSII to PSI fluorescence ratio reflects that the dark-adapted iron-replete cells are in State 2 while the dark-adapted ironstarved cells are in State 1 (Ivanov et al, 2006).…”

“…The 685-nm fluorescence emission reflects the presence of PSI-less IsiA aggregates (Yeremenko et al, 2004;Wilson et al, 2007) (Fig.4.6.A The typical shift of the PSI-related peak from 730 nm to 725 nm was observed in both strains and is related to IsiA complexes surrounding PSI (Burnap et al, 1993;Falk et al, 1995;Park et al, 1999) (Fig.4.6.A).…”