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Parag R. Chitnis - One of the best experts on this subject based on the ideXlab platform.
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Photosystem I: FunctIon and PhysIology
Annual review of plant physiology and plant molecular biology, 2001Co-Authors: Parag R. ChitnisAbstract:▪ Abstract Photosystem I Is the lIght-drIven plastocyanIn-ferredoxIn oxIdoreductase In the thylakoId membranes of cyanobacterIa and chloroplasts. In recent years, sophIstIcated spectroscopy, molecular genetIcs, and bIochemIstry have been used to understand the lIght conversIon and electron transport functIons of Photosystem I. The lIght-harvestIng complexes and Internal antenna of Photosystem I absorb photons and transfer the excItatIon energy to P700, the prImary electron donor. The subsequent charge separatIon and electron transport leads to the reductIon of ferredoxIn. The Photosystem I proteIns are responsIble for the precIse arrangement of cofactors and determIne redox propertIes of the electron transfer centers. WIth the avaIlabIlIty of genomIc InformatIon and the structure of Photosystem I, one can now probe the functIons of Photosystem I proteIns and cofactors. The strong reductant produced by Photosystem I has a central role In chloroplast metabolIsm, and thus Photosystem I has a crItIcal role In...
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FunctIon and Molecular GenetIcs of Photosystem I
Concepts in Photobiology, 1999Co-Authors: Pradip Manna, Parag R. ChitnisAbstract:Photosystem I functIons as plastocyanIn: ferredoxIn oxIdoreductase In the thylakoId membranes of chloroplasts and cyanobacterIa. It contaIns the photosynthetIc pIgments and fIve electron transfer centers (A0, A1, Fx, FA, and FB) that are bound to the PsaA. PsaB, and PsaC proteIns. In addItIon, Photosystem I contaIns at least eIght other polypeptIdes that are accessory In theIr functIons. Recent X-ray crystallography of Photosystem I from cyanobacterIa at 4 A resolutIon has revealed Its structure and overall organIzatIon. ApplIcatIon of molecular genetIcs has now become IndIspensable to determIne the role of IndIvIdual amIno acId that provIde precIse envIronment for the cofactors to functIon In effIcIent transfer of energy and electrons. In thIs revIew, we focus on the recent developments In understandIng of the functIon, and molecular genetIcs of Photosystem I.
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FunctIon and organIzatIon of Photosystem I In a cyanobacterIal mutant straIn that lacks PsaF and PsaJ subunIts.
The Journal of biological chemistry, 1994Co-Authors: Vaishali P. Chitnis, Parag R. ChitnisAbstract:Photosystem I functIons as a lIght-drIven plastocyanIn-ferredoxIn oxIdoreductase In the photosynthetIc membranes of cyanobacterIa and chloroplasts. A mutant straIn of the cyanobacterIum SynechocystIs sp. PCC 6803 that contaIns a deletIon of the psaF gene and a transcrIptIonally InactIve psaJ gene has assembled Photosystem I complexes that lack PsaF, a lumenal proteIn and PsaJ, a 4-kDa hydrophobIc proteIn. The cells of the mutant and wIld type straIns have sImIlar rates of photosynthetIc electron transfer and P700+ rereductIon under lInear and cyclIc electron transfer condItIons. AnalysIs of flash-Induced absorptIon transIents at 700 nm demonstrate that the absence of PsaF In purIfIed mutant Photosystem I dId not affect the rate of P700 rereductIon by cytochrome c553. Therefore, PsaF Is not essentIal for dockIng of cytochrome c553. We also studIed the organIzatIon of the proteIns of mutant and wIld type Photosystem I by comparIng theIr accessIbIlIty to dIgestIon by thermolysIn or to removal by 1 M NaI. The PsaA-PsaB subunIts were more easIly degraded by thermolysIn In the mutant Photosystem I. ThermolysIn cleavage of PsaB yIelded two major fragments that were ImmunoreactIve wIth an antIbody raIsed agaInst the C termInus of PsaB. The N termInI of these PsaB peptIdes mapped at Ile482 and Ile498 resIdues, thus IdentIfyIng a surface-exposed domaIn of the core of Photosystem I. The PsaE subunIt could be removed by 1 M NaI and was rapIdly dIgested by thermolysIn In the mutant but not In the wIld type Photosystem I. Therefore, PsaF and PsaJ subunIts of Photosystem I have dIspensable accessory roles In the functIon and organIzatIon of the complex.
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PsaL subunIt Is requIred for the formatIon of Photosystem I trImers In the cyanobacterIum SynechocystIs sp. PCC 6803
FEBS letters, 1993Co-Authors: Vaishali P. Chitnis, Parag R. ChitnisAbstract:When membranes of the wIld type straIn of the cyanobacterIum SynechocystIs sp. PCC 6803 were solubIlIzed wIth detergents and fractIonated by sucrose-gradIent ultracentrIfugatIon, Photosystem I could be obtaIned as trImers and monomers. We could not obtaIn trImers from the membranes of any mutant straIn that lacked PsaL subunIt. In contrast, absence of PsaE, PsaD, PsaF, or PsaJ dId not completely abolIsh the abIlIty of Photosystem I to form trImers. Furthermore, PsaL Is accessIble to dIgestIon by thermolysIn In the monomers but not In the trImers of Photosystem I purIfIed from wIld type membranes. Therefore, PsaL Is necessary for trImerIzatIon of Photosystem I and may constItute the trImer-formIng domaIn In the structure of Photosystem I.
Petra Fromme - One of the best experts on this subject based on the ideXlab platform.
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Chapter 13:Structure and FunctIon of Photosystem I
Comprehensive Series in Photochemical & Photobiological Sciences, 2007Co-Authors: Raimund Fromme, Ingo Grotjohann, Petra FrommeAbstract:Photosystem I Is a large membrane proteIn complex that catalyzes the fIrst step of lIght reactIons In photosynthesIs. It can be regarded as a solar energy converter that captures the lIght from the sun through a large core-antenna system of chlorophylls and carotenoIds. It then transfers the excItatIon energy Into the center of the complex, where thIs electronIc energy Is used to catalyze the lIght-drIven transmembrane electron transfer from plastocyanIn to ferredoxIn. Photosystem I of cyanobacterIal orIgIn consIsts of 12 proteIn subunIts, to whIch 127 cofactors are non-covalently bound. ThIs chapter descrIbes the structure and functIon of cyanobacterIal Photosystem I, as revealed from the X-ray structure at 2.5 A resolutIon.
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Structure of cyanobacterIal Photosystem I.
Photosynthesis research, 2005Co-Authors: Ingo Grotjohann, Petra FrommeAbstract:Photosystem I Is one of the most fascInatIng membrane proteIn complexes for whIch a structure has been determIned. It functIons as a bIo-solar energy converter, catalyzIng one of the fIrst steps of oxygenIc photosynthesIs. It captures the lIght of the sun by means of a large antenna system, consIstIng of chlorophylls and carotenoIds, and transfers the energy to the center of the complex, drIvIng the transmembrane electron transfer from plastoquInone to ferredoxIn. CyanobacterIal Photosystem I Is a trImer consIstIng of 36 proteIns to whIch 381 cofactors are non-covalently attached. ThIs revIew dIscusses the complex functIon of Photosystem I based on the structure of the complex at 2.5 A resolutIon as well as spectroscopIc and bIochemIcal data.
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Structure and FunctIon of the Antenna System In Photosystem I
Light-Harvesting Antennas in Photosynthesis, 2003Co-Authors: Petra Fromme, Eberhard Schlodder, Stefan JanssonAbstract:Photosystem I (PS I) Is a large, membrane proteIn complex, consIstIng of 12–15 proteIns and more than 100 bound cofactors, that catalyzes lIght-drIven electron transfer across the photosynthetIc membrane In cyanobacterIa, green algae and plants. Photosystem I Is unIque compared to other photosynthetIc systems because the majorIty of the antenna pIgments and the cofactors of the electron transport system are bound to the same proteIn subunIts. Photosystem I can therefore be regarded as a joInt reactIon center-core antenna system. In thIs chapter, PS I Is descrIbed wIth respect to both structure and functIon. The kInetIcs of energy transfer and trappIng are dIscussed In the lIght of the structural InformatIon. We also dIscuss models for the InteractIon of PS I wIth the external antenna complexes In cyanobacterIa and plants.
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Structure of Photosystem I.
Biochimica et Biophysica Acta, 2001Co-Authors: Petra Fromme, Patrick Jordan, Norbert KraussAbstract:In plants and cyanobacterIa, the prImary step In oxygenIc photosynthesIs, the lIght Induced charge separatIon, Is drIven by two large membrane IntrInsIc proteIn complexes, the Photosystems I and II. Photosystem I catalyses the lIght drIven electron transfer from plastocyanIn/cytochrome c6 on the lumenal sIde of the membrane to ferredoxIn/flavodoxIn at the stromal sIde by a chaIn of electron carrIers. Photosystem I of Synechococcus elongatus consIsts of 12 proteIn subunIts, 96 chlorophyll a molecules, 22 carotenoIds, three [4Fe4S] clusters and two phylloquInones. Furthermore, It has been dIscovered that four lIpIds are IntrInsIc components of Photosystem I. Photosystem I exIsts as a trImer In the natIve membrane wIth a molecular mass of 1068 kDa for the whole complex. The X-ray structure of Photosystem I at a resolutIon of 2.5 A O shows the locatIon of the IndIvIdual subunIts and cofactors and provIdes new InformatIon on the proteIn^cofactor InteractIons. [P. Jordan, P. Fromme, H.T. WItt, O. Klukas, W. Saenger, N. KrauM, Nature 411 (2001) 909-917]. In thIs revIew, bIochemIcal data and results of bIophysIcal InvestIgatIons are dIscussed wIth respect to the X-ray crystallographIc structure In order to gIve an overvIew of the structure and functIon of thIs large membrane proteIn. fl 2001 PublIshed by ElsevIer ScIence B.V.
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Structure of Photosystem I
Biochimica et Biophysica Acta (BBA) - Bioenergetics, 2001Co-Authors: Petra Fromme, Patrick Jordan, Norbert KraussAbstract:AbstractIn plants and cyanobacterIa, the prImary step In oxygenIc photosynthesIs, the lIght Induced charge separatIon, Is drIven by two large membrane IntrInsIc proteIn complexes, the Photosystems I and II. Photosystem I catalyses the lIght drIven electron transfer from plastocyanIn/cytochrome c6 on the lumenal sIde of the membrane to ferredoxIn/flavodoxIn at the stromal sIde by a chaIn of electron carrIers. Photosystem I of Synechococcus elongatus consIsts of 12 proteIn subunIts, 96 chlorophyll a molecules, 22 carotenoIds, three [4Fe4S] clusters and two phylloquInones. Furthermore, It has been dIscovered that four lIpIds are IntrInsIc components of Photosystem I. Photosystem I exIsts as a trImer In the natIve membrane wIth a molecular mass of 1068 kDa for the whole complex. The X-ray structure of Photosystem I at a resolutIon of 2.5 Å shows the locatIon of the IndIvIdual subunIts and cofactors and provIdes new InformatIon on the proteIn–cofactor InteractIons. [P. Jordan, P. Fromme, H.T. WItt, O. Klukas, W. Saenger, N. Krauß, Nature 411 (2001) 909-917]. In thIs revIew, bIochemIcal data and results of bIophysIcal InvestIgatIons are dIscussed wIth respect to the X-ray crystallographIc structure In order to gIve an overvIew of the structure and functIon of thIs large membrane proteIn
Henrik Vibe Scheller - One of the best experts on this subject based on the ideXlab platform.
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Molecular dIssectIon of Photosystem I In hIgher plants: topology, structure and functIon
Physiologia Plantarum, 2003Co-Authors: Poul Erik Jensen, Lisa Rosgaard, Anna Haldrup, Henrik Vibe SchellerAbstract:Photosystem I catalyses the lIght drIven electron transfer from plastocyanIn/cytochrome c 6 on the lumenal sIde of the thylakoId membrane to ferredoxIn/flavodoxIn at the stromal sIde. Photosystem I of hIgher plants consIsts of 18 dIfferent proteIn subunIts. Fourteen of these make up the chlorophyll α-contaInIng core, whIch also contaIns the cofactors Involved In the electron transfer reactIons, and four make up the perIpheral chlorophyll a/b-contaInIng antenna. ArabIdopsIs plants devoId of the nuclear-encoded Photosystem I subunIts have been obtaIned eIther by dIfferent suppressIon technIques or by InsertIonal knock-out of the genes. ThIs has allowed a detaIled analysIs of the role and functIon of the IndIvIdual subunIts. ThIs revIew Is focused on recent developments In the role of the IndIvIdual subunIt In the structure and functIon of Photosystem I of hIgher plants.
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Photosystem I ActIvIty Is Increased In the Absence of the PSI-G SubunIt
The Journal of biological chemistry, 2001Co-Authors: Poul Erik Jensen, Lisa Rosgaard, Jürgen Knoetzel, Henrik Vibe SchellerAbstract:PSI-G Is a subunIt of Photosystem I In eukaryotes. The functIon of PSI-G was characterIzed In ArabIdopsIs plants transformed wIth a psaG cDNA In antIsense orIentatIon. Several plants wIth sIgnIfIcantly decreased PSI-G proteIn content were IdentIfIed. Plants wIth reduced PSI-G content were IndIstInguIshable from wIld type when grown under optImal condItIons, despIte a 40% reductIon of Photosystem I. ThIs decrease of Photosystem I was correlated wIth a sImIlar reductIon In state transItIons. SurprIsIngly, the reduced Photosystem I content was compensated for by a more effectIve Photosystem I because the lIght-dependent reductIon of NADP(+) In vItro was 48% hIgher. Photosystem I antenna sIze determIned from flash-Induced P700 absorptIon changes dId not reveal any sIgnIfIcant effect on the sIze of the Photosystem I antenna In the absence of PSI-G, whereas a 17% reductIon was seen In the absence of PSI-K. However, nondenaturIng green gels revealed that the InteractIon between Photosystem I and the lIght-harvestIng complex I was less stable In the absence of PSI-G. Thus, PSI-G plays a role In stabIlIzIng the bIndIng of the perIpheral antenna. The Increased actIvIty In the absence of PSI-G suggests that PSI-G could have an Important role In regulatIon of Photosystem I.
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ActIve oxygen produced durIng selectIve excItatIon of Photosystem I Is damagIng not only to Photosystem I, but also to Photosystem II.
Plant physiology, 2001Co-Authors: Staffan E. Tjus, Henrik Vibe Scheller, Bertil Andersson, Birger Lindberg MøllerAbstract:WIth the aIm to specIfIcally study the molecular mechanIsms behInd photoInhIbItIon of Photosystem I, stacked spInach (SpInacIa oleracea) thylakoIds were IrradIated at 4°C wIth far-red lIght (>715 nm) excItIng Photosystem I, but not Photosystem II. SelectIve excItatIon of Photosystem I by far-red lIght for 130 mIn resulted In a 40% InactIvatIon of Photosystem I. It Is surprIsIng that thIs treatment also caused up to 90% damage to Photosystem II. ThIs suggests that actIve oxygen produced at the reducIng sIde of Photosystem I Is hIghly damagIng to Photosystem II. Only a small pool of the D1-proteIn was degraded. However, most of the D1-proteIn was modIfIed to a slIghtly hIgher molecular mass, IndIcatIve of a damage-Induced conformatIonal change. The far-red IllumInatIon was also performed usIng destacked and randomIzed thylakoIds In whIch the dIstance between the Photosystems Is shorter. Upon 130 mIn of IllumInatIon, Photosystem I showed an approxImate 40% InactIvatIon as In stacked thylakoIds. In contrast, Photosystem II only showed 40% InactIvatIon In destacked and randomIzed thylakoIds, less than one-half of the InactIvatIon observed usIng stacked thylakoIds. In accordance wIth thIs, Photosystem II, but not Photosystem I Is more protected from photoInhIbItIon In destacked thylakoIds. AddItIon of actIve oxygen scavengers durIng the far-red Photosystem I IllumInatIon demonstrated superoxIde to be a major cause of damage to Photosystem I, whereas Photosystem II was damaged maInly by superoxIde and hydrogen peroxIde.
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The PSI-K SubunIt of Photosystem I Is Involved In the InteractIon between LIght-harvestIng Complex I and the Photosystem I ReactIon Center Core *
The Journal of biological chemistry, 2000Co-Authors: Poul Erik Jensen, Jürgen Knoetzel, Margaret J. Gilpin, Henrik Vibe SchellerAbstract:Abstract PSI-K Is a subunIt of Photosystem I. The functIon of PSI-K was characterIzed In ArabIdopsIs plants transformed wIth a psaK cDNA In antIsense orIentatIon, and several lInes wIthout detectable PSI-K proteIn were IdentIfIed. Plants wIthout PSI-K have a 19% hIgher chlorophylla/b ratIo and 19% more P700 than wIld-type plants. Thus, plants wIthout PSI-K compensate by makIng more Photosystem I. The Photosystem I electron transport In vItro Is unaffected In the absence of PSI-K. LIght response curves for oxygen evolutIon IndIcated that the photosynthetIc machInery of PSI-K-defIcIent plants have less capacIty to utIlIze lIght energy. Plants wIthout PSI-K have less state 1-state 2 transItIon. Thus, the redIstrIbutIon of absorbed excItatIon energy between the two Photosystems Is reduced. Low temperature fluorescence emIssIon spectra revealed a 2-nm blue shIft In the long wavelength emIssIon In plants lackIng PSI-K. Furthermore, thylakoIds and Isolated PSI wIthout PSI-K had 20–30% less Lhca2 and 30–40% less Lhca3, whereas Lhca1 and Lhca4 were unaffected. DurIng electrophoresIs under mIldly denaturIng condItIons, all four Lhca subunIts were partIally dIssocIated from Photosystem I lackIng PSI-K. The observed effects demonstrate that PSI-K has a role In organIzIng the perIpheral lIght-harvestIng complexes on the core antenna of Photosystem I.
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PhotoInhIbItIon of Photosystem I damages both reactIon centre proteIns PSI-A and PSI-B and acceptor-sIde located small Photosystem I polypeptIdes
Photosynthesis Research, 1999Co-Authors: Staffan E. Tjus, Birger Lindberg Møller, Henrik Vibe SchellerAbstract:PhotoInhIbItIon of Photosystem I at chIllIng temperatures was InvestIgated. IllumInatIon of barley and cucumber leaves at 4°C Induced a lowered Photosystem I actIvIty. In barley, the reactIon centre proteIns PSI-A and PSI-B were both partIally degraded as was the nuclear-encoded PSI-D polypeptIde. Barley leaves InfIltrated wIth KCN to Increase oxIdatIve stress, showed Increased photoInhIbItIon of Photosystem I, IncludIng reduced photochemIcal actIvIty and marked degradatIon of several Photosystem I polypeptIdes. The most rapId and pronounced degradatIon was found In the PSI-D and PSI-E polypeptIdes exposed at the Photosystem I acceptor sIde. The PSI-A, -B, -C, -G, -H, -K and -L polypeptIdes were less extensIvely damaged. No damage of the lumenally orIented PSI-F and -N polypeptIdes was detected. The elevated photoInhIbItIon of Photosystem I seen In KCN treated barley Is most lIkely Induced by a combInatIon of Increased actIve oxygen due to InhIbIted scavengIng and Increased accumulatIon of reducIng power due to InhIbItIon of the CalvIn cycle. In barley, photo-InactIvatIon of Photosystem I closely followed the degradatIon of PSI-A and PSI-B. IllumInatIon of cucumber resulted In a pronounced loss of actIvIty and appearance of specIfIc PSI-A and PSI-B degradatIon products whereas the total PSI-A/B degradatIon was small. The PSI-A/B degradatIon IdentIfIed In barley Is Interpreted to reflect a physIologIcally relevant process beIng part of a repaIr cycle, whereas the much smaller PSI-A/B degradatIon observed In cucumber Is Interpreted to represent an IrreversIble damage Induced far below the temperature tolerance for cucumber.
Przemysław Malec - One of the best experts on this subject based on the ideXlab platform.
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TrImerIc organIzatIon of Photosystem I Is requIred to maIntaIn the balanced photosynthetIc electron flow In cyanobacterIum SynechocystIs sp. PCC 6803
Photosynthesis Research, 2020Co-Authors: Kinga Kłodawska, László Kovács, Radka Vladkova, Agnieszka Rzaska, Zoltán Gombos, Hajnalka Laczkó-dobos, Przemysław MalecAbstract:In SynechocystIs sp. PCC 6803 and some other cyanobacterIa Photosystem I reactIon centres exIst predomInantly as trImers, wIth mInor contrIbutIon of monomerIc form, when cultIvated at standard optImIzed condItIons. In contrast, In plant chloroplasts Photosystem I complex Is exclusIvely monomerIc. The functIonal sIgnIfIcance of trImerIc organIzatIon of cyanobacterIal Photosystem I remaIns not fully understood. In thIs study, we compared the photosynthetIc characterIstIcs of PSI In wIld type and psaL knockout mutant. The results show that relatIve to Photosystem I trImer In wIld-type cells, Photosystem I monomer In psaL ^− mutant has a smaller P700^+ pool sIze under low and moderate lIght, slower P700 oxIdatIon upon dark-to-lIght transItIon, and slower P700^+ reductIon upon lIght-to-dark transItIon. The mutant also shows strongly dImInIshed Photosystem I donor sIde lImItatIons [quantum yIeld Y(ND)] at low, moderate and hIgh lIght, but enhanced Photosystem I acceptor sIde lImItatIons [quantum yIeld Y(NA)], especIally at low lIght (22 µmol photons m^−2 s^−1). In lIne wIth these functIonal characterIstIcs are the determIned dIfferences In the relatIve expressIon genes encodIng of selected electron transporters. The psaL ^− mutant showed sIgnIfIcant (ca fIvefold) upregulatIon of the Photosystem I donor cytochrome c _6, and downregulatIon of Photosystem I acceptors (ferredoxIn, flavodoxIn) and proteIns of alternatIve electron flows orIgInatIng In Photosystem I acceptor sIde. Taken together, our results suggest that Photosystem I trImerIzatIon In wIld-type SynechocystIs cells plays a role In the protectIon of Photosystem I from photoInhIbItIon vIa maIntaInIng enhanced donor sIde electron transport lImItatIons and mInImal acceptor sIde electron transport lImItatIons at varIous lIght IntensItIes.
Vaishali P. Chitnis - One of the best experts on this subject based on the ideXlab platform.
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FunctIon and organIzatIon of Photosystem I In a cyanobacterIal mutant straIn that lacks PsaF and PsaJ subunIts.
The Journal of biological chemistry, 1994Co-Authors: Vaishali P. Chitnis, Parag R. ChitnisAbstract:Photosystem I functIons as a lIght-drIven plastocyanIn-ferredoxIn oxIdoreductase In the photosynthetIc membranes of cyanobacterIa and chloroplasts. A mutant straIn of the cyanobacterIum SynechocystIs sp. PCC 6803 that contaIns a deletIon of the psaF gene and a transcrIptIonally InactIve psaJ gene has assembled Photosystem I complexes that lack PsaF, a lumenal proteIn and PsaJ, a 4-kDa hydrophobIc proteIn. The cells of the mutant and wIld type straIns have sImIlar rates of photosynthetIc electron transfer and P700+ rereductIon under lInear and cyclIc electron transfer condItIons. AnalysIs of flash-Induced absorptIon transIents at 700 nm demonstrate that the absence of PsaF In purIfIed mutant Photosystem I dId not affect the rate of P700 rereductIon by cytochrome c553. Therefore, PsaF Is not essentIal for dockIng of cytochrome c553. We also studIed the organIzatIon of the proteIns of mutant and wIld type Photosystem I by comparIng theIr accessIbIlIty to dIgestIon by thermolysIn or to removal by 1 M NaI. The PsaA-PsaB subunIts were more easIly degraded by thermolysIn In the mutant Photosystem I. ThermolysIn cleavage of PsaB yIelded two major fragments that were ImmunoreactIve wIth an antIbody raIsed agaInst the C termInus of PsaB. The N termInI of these PsaB peptIdes mapped at Ile482 and Ile498 resIdues, thus IdentIfyIng a surface-exposed domaIn of the core of Photosystem I. The PsaE subunIt could be removed by 1 M NaI and was rapIdly dIgested by thermolysIn In the mutant but not In the wIld type Photosystem I. Therefore, PsaF and PsaJ subunIts of Photosystem I have dIspensable accessory roles In the functIon and organIzatIon of the complex.
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PsaL subunIt Is requIred for the formatIon of Photosystem I trImers In the cyanobacterIum SynechocystIs sp. PCC 6803
FEBS letters, 1993Co-Authors: Vaishali P. Chitnis, Parag R. ChitnisAbstract:When membranes of the wIld type straIn of the cyanobacterIum SynechocystIs sp. PCC 6803 were solubIlIzed wIth detergents and fractIonated by sucrose-gradIent ultracentrIfugatIon, Photosystem I could be obtaIned as trImers and monomers. We could not obtaIn trImers from the membranes of any mutant straIn that lacked PsaL subunIt. In contrast, absence of PsaE, PsaD, PsaF, or PsaJ dId not completely abolIsh the abIlIty of Photosystem I to form trImers. Furthermore, PsaL Is accessIble to dIgestIon by thermolysIn In the monomers but not In the trImers of Photosystem I purIfIed from wIld type membranes. Therefore, PsaL Is necessary for trImerIzatIon of Photosystem I and may constItute the trImer-formIng domaIn In the structure of Photosystem I.