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.
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.
Ivanov
Alexander G
AG
Department of Biology and The Biotron, University of Western Ontario, London, Ontario, Canada N6A 5B7.
Krol
Marianna
M
Sveshnikov
Dmitry
D
Selstam
Eva
E
Sandström
Stefan
S
Koochek
Maryam
M
Park
Youn-Il
YI
Vasil'ev
Sergej
S
Bruce
Doug
D
Oquist
Gunnar
G
Huner
Norman P A
NP
D001426:Bacterial Proteins / Q000378:metabolism* / Q000502:physiology; D007150:Immunohistochemistry; D007501:Iron / Q000502:physiology*; D045342:Light-Harvesting Protein Complexes / Q000378:metabolism / Q000502:physiology; D050356:Lipid Metabolism; D009252:NADPH Dehydrogenase / Q000378:metabolism; D010084:Oxidation-Reduction; D044367:Phase Transition; D010715:Phosphatidylglycerols / Q000378:metabolism; D045331:Photosystem I Protein Complex / Q000378:metabolism*; D045332:Photosystem II Protein Complex / Q000378:metabolism; D046940:Synechococcus / Q000201:enzymology / Q000378:metabolism*
D001426:Bacterial Proteins; D045342:Light-Harvesting Protein Complexes; D010715:Phosphatidylglycerols; D045331:Photosystem I Protein Complex; D045332:Photosystem II Protein Complex; C082500:photosystem I, psaL subunit; C422369:photosystem II, psbA subunit; C076726:chlorophyll A binding protein CP43, Cyanobacteria; D007501:Iron; D009252:NADPH Dehydrogenase