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The induction of the isiA (CP43') protein in iron-stressed cyanobacteria is accompanied by the formation of a ring of 18 CP43' proteins around the photosystem I (PSI) trimer and is thought to increase the absorption cross section of PSI within the CP43'-PSI supercomplex. In contrast to these in vitro studies, our in vivo measurements failed to demonstrate any increase of the PSI absorption cross section in two strains (Synechococcus sp. PCC 7942 and Synechocystis sp. PCC 6803) of iron-stressed cells. We report that iron-stressed cells exhibited a reduced capacity for state transitions and limited dark reduction of the plastoquinone pool, which accounts for the increase in PSII-related 685 nm chlorophyll fluorescence under iron deficiency. This was accompanied by lower abundance of the NADP-dehydrogenase complex and the PSI-associated subunit PsaL, as well as a reduced amount of phosphatidylglycerol. Nondenaturating polyacrylamide gel electrophoresis separation of the chlorophyll-protein complexes indicated that the monomeric form of PSI is favored over the trimeric form of PSI under iron stress. Thus, we demonstrate that the induction of CP43' does not increase the PSI functional absorption cross section of whole cells in vivo, but rather, induces monomerization of PSI trimers and reduces the capacity for state transitions. We discuss the role of CP43' as an effective energy quencher to photoprotect PSII and PSI under unfavorable environmental conditions in cyanobacteria in vivo.
Among various structural (Sherman and Sherman, 1983) and functional responses of cyanobacterial cells to growth under iron-limited conditions (Öquist, 1974a, 1974b; Guikema and Sherman, 1983; Ivanov et al., 2000; Sandström et al., 2001, 2002; Michel and Pistorius, 2004), one of the most specific is the appearance of a chlorophyll (Chl)-protein complex associated with the isiA gene product CP43' (Pakrasi et al., 1985a, 1985b; Riethman and Sherman, 1988; Burnap et al., 1993). Since CP43' possesses high homology to the ChI a-binding protein CP43 of PSII core (Burnap et al., 1993; Falk et al., 1995), the increase of the low temperature (77 K) PSII-related ChI fluorescence peak at 685 nm in iron-stressed cells was initially linked to the induction of the CP43' polypeptide (Burnap et al., 1993; FaIk et al., 1995; Park et al., 1999). It was suggested that CP43' could replace CP43, thus acting as an alternative antenna complex for...