Ivanov AG, Krol M., Sveshnikov D, Selstam E, Sandström S, Koochek M., Park Y-I, Vasil'ev S, Bruce D, Öquist G, Huner NPA
Iron deficiency in cyanobacteria causes monomerization of photosystem I trimers and reduces the capacity for state transitions and the effective absorption cross section of photosystem I in vivo
Plant Physiology: 2006 141:1436-1445

The induction of the isiA (CP43') protein in iron-stressed cyanobacteriais accompanied by the formation of a ring of 18 CP43' proteinsaround the photosystem I (PSI) trimer and is thought to increasethe absorption cross section of PSI within the CP43'-PSI supercomplex.In contrast to these in vitro studies, our in vivo measurementsfailed to demonstrate any increase of the PSI absorption crosssection in two strains (Synechococcus sp. PCC 7942 and Synechocystissp. PCC 6803) of iron-stressed cells. We report that iron-stressedcells exhibited a reduced capacity for state transitions andlimited dark reduction of the plastoquinone pool, which accountsfor the increase in PSII-related 685 nm chlorophyll fluorescenceunder iron deficiency. This was accompanied by lower abundanceof the NADP-dehydrogenase complex and the PSI-associated subunitPsaL, as well as a reduced amount of phosphatidylglycerol. Nondenaturatingpolyacrylamide gel electrophoresis separation of the chlorophyll-proteincomplexes indicated that the monomeric form of PSI is favoredover the trimeric form of PSI under iron stress. Thus, we demonstratethat the induction of CP43' does not increase the PSI functionalabsorption cross section of whole cells in vivo, but rather,induces monomerization of PSI trimers and reduces the capacityfor state transitions. We discuss the role of CP43' as an effectiveenergy quencher to photoprotect PSII and PSI under unfavorableenvironmental conditions in cyanobacteria in vivo.

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