N-terminal amino acid sequence analysis of the subunits of pea photosystem I

Structure of cyanobacterial photosystem I

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.

Modified ligands to FA and FB in photosystem I. Proposed chemical rescue of a [4Fe-4S] cluster with an external thiolate in alanine, glycine, and serine mutants of PsaC

The FB and FA electron acceptors in Photosystem I (PS I) are [4Fe-4S] clusters ligated by cysteines provided by PsaC. In a previous study (Mehari, T., Qiao, F., Scott, M. P., Nellis, D., Zhao, J., Bryant, D., and Golbeck, J. H. (1995) J. Biol. Chem. 270, 28108-28117), we showed that when cysteines 14 and 51 were replaced with serine or alanine, the free proteins contained a S = 1/2, [4Fe-4S] cluster at the unmodified site and a mixed population of S = 1/2, [3Fe-4S] and S = 3/2, [4Fe-4S] clusters at the modified site. We show here that these mutant PsaC proteins can be rebound to P700-FX cores, resulting in fully functional PS I complexes. The low temperature EPR spectra of the C14XPsaC.PS I complexes (where X = S, A, or G) show the photoreduction of a wild-type FA cluster and a modified FB' cluster, the latter with g values of 2.115, 1.899, and 1.852 and linewidths of 110, 70, and 85 MHz. Since neither alanine nor glycine contains a suitable side group, an external thiolate provided by beta-mercaptoethanol has likely been recruited to supply the requisite ligand to the [4Fe-4S] cluster. The EPR spectrum of the C51SPsaC.PS I complex differs from that of the C51APsaC.PS I or C51GPsaC.PS I complexes by the presence of an additional set of resonances, which may be derived from the serine oxygen-ligated cluster. In all other mutant PS I complexes, a wild-type spin-coupled interaction spectrum appears when FA and FB are simultaneously reduced. Single turnover flash studies indicate approximately 50% efficient electron transfer to FA/FB in the C14SPsaC.PS I, C51SPsaC.PS I, C14GPsaC.PS I, and C51GPsaC.PS I mutants and less than 40% in the C14APsaC.PS I and C51APsaC.PS I mutants, compared with approximately 76% in the PS I core reconstructed with wild-type PsaC. These data are consistent with the measurements of the rates of cytochrome c6-NADP+ reductase activity, indicating lower rates in the alanine mutants. It is proposed that the chemical rescue of a [4Fe-4S] cluster with a recruited external thiolate at the modified site allows the mutant PsaC proteins to rebind to PS I and to function in forward electron transfer.

Nucleotide sequence of the psaD gene from the thermophilic cyanobacterium Synechococcus vulcanus

The nucleotide sequence was determined for the psaD gene of a thermophilic cyanobacterium, Synechococcus vulcanus, which encoded the PsaD subunit (Subunit II) of the Photosystem I reaction center complex. Except for some differences in the peripherals, the nucleotide sequence of the gene encoding PsaD was identical to that of another thermophilic cyanobacterium Synechococcus elongatus reported previously. Relationship between these primary structures and thermostability was also discussed.

Purification and membrane topology of PSI-D and PSI-E, two subunits of the photosystem I reaction center

Structural studies have been conducted on polypeptides PSI-D and PSI-E, which are extrinsic but firmly bound to the photosystem I reaction center. These subunits are predicted to be involved in the correct interaction with soluble electron acceptor(s), like ferredoxin. We designed an original method to extract both polypeptides directly from thylakoid membranes and to purify them: a stepwise extraction with NaSCN followed by size fractionation and reverse-phase HPLC. Investigation of the in situ topology of PSI-D and PSI-E was undertaken using monoclonal antibody binding, controlled proteolysis, peptide sequencing and electron microscopy. The precise identification of numerous proteolytic sites indicates that the entire N-terminal regions of PSI-E (up to Glu15) and PSI-D (up to Lys15) are exposed to the medium. Partial mapping of the exposed epitopes was possible using purified fragments of each polypeptide. In the case of PSI-E, this mapping confirmed the accessibility of the N-terminal part, and suggested the need for another exposed sequence, probably located after Met39 in the second half of the protein. For PSI-D, this mapping revealed that the sequence between Met74 and Met140, including the most basic amino acid clusters, is also partly accessible. These experiments provide the first detailed informations, although still partial, on the topology of these polypeptides. They give a preliminary basis for hypotheses concerning the sites of interaction with the soluble counterparts.

Characterization of genes that encode subunits of cucumber PS I complex by N-terminal sequencing

N-terminal amino acid sequencing was carried out to characterize the genes of the cucumber PS I complex (PSI-100) that contains eight polypeptides and catalyzes the light-dependent transfer of electrons from plastocyanin to ferredoxin. The genes of all subunits except the 17.5 kDa polypeptide in PSI-100 have been identified. These are psaA/psaB (65/63 kDa), psaD (20 kDa), psaE (19.5 kDa), psaF (18.5 kDa), psaH (7.6 kDa), and psaC (5.8 kDa). The 17.5 kDa polypeptide is a new protein and is designated tentatively as the gene product of psaM. N-terminal amino-acid sequencing indicated the presence of two polypeptides in the 7.6 kDa band. One of these is the gene product of psaH and is essential for the activity of the PS I complex, and the other one is as yet unrecognized and largely depleted in the PSI-100 complex. Gene products of psaG, psaI, and psaK, which have been proposed as the components of PS I complex, are not involved in the PSI-100 complex, but are involved in the PS I complex (PSI-200), which contains 120 chlorophyll per reaction center chlorophyll (P700) and light-harvesting chlorophyll a/b protein complexes. Three polypeptides (26,23 and 22.5 kDa) are not involved in the PSI-100 and are assigned as the apo-protein of light-harvesting chlorophyll a/b protein complexes.

Correlation of some published amino acid sequences for photosystem I polypeptides to a 17 kDa LHCI pigment-protein and to subunits III and IV of the core complex

Photosystem I (PSI) in barley consists of at least 11 polypeptides of which three have apparent sizes of 15-19 kDa. Two of these polypeptides (subunits III and IV) are constituents of the core complex (CCI), the third is a component of the light-harvesting complex (LHCI). After fractionation of PSI into its CCI and LHCI components, each of the polypeptides has been isolated and its N-terminal region sequenced. We conclude that the gene sequence published for subunit IV of spinach [(1988) FEBS Lett. 237, 108-112] is not that of subunit IV but rather that of the 17 kDA LHCIc pigment protein. We confirm that the published sequence for subunit III [(1988) Curr. Genet. 14, 511-518] is indeed that of subunit III; seemingly conflicting identifications, based on apparent sizes on SDS-PAGE, of which polypeptides are subunits III and IV are probably explained by subunit III's electrophoretic migration rate being dependent on the solvent.

Isolation and characterization of cDNA clones encoding photosystem I subunits with molecular masses 11.0, 10.0 and 8.4 kDa from Chlamydomonas reinhardtii

cDNA clones encoding three photosystem I subunits of Chlamydomonas reinhardtii with apparent molecular masses 13, 5 and 3 kDa (thylakoid polypeptides 28, 35 and 37; P28, P35 and P37, respectively) were isolated using gene specific oligonucleotides as probes. The sequences of these oligonucleotides were deduced from the N-terminal amino acid sequences of the proteins. The cDNAs were sequenced and used to probe Southern and Northern blots. The Southern blot analysis indicates that the proteins are encoded by single-copy genes. The mRNA sizes of the three components are 960 (P28), 1120 (P35) and 790 (P37) nucleotides. Comparison between the open reading frames of the cDNAs and the N-terminal amino acid sequences of the proteins indicates that the nascent polypeptides possess N-terminal transit sequences that are removed to give mature proteins of 11.0 (P28), 10.0 (P35) and 8.4 (P37) kDa. Analysis of the deduced protein sequences suggests that P28 and P35 are extrinsic membrane proteins and that P37 spans the thylakoid membrane. All three proteins have short transit peptides that probably route them to the stromal side of the thylakoid membrane.

Identification of photosystem I components from the cyanobacterium, Synechococcus vulcanus by N-terminal sequencing

The photosystem I core complex isolated from a thermophilic cyanobacterium, Synechococcus vulcanus, is composed of eight low-molecular-mass proteins of 18, 14, 12, 9.5, 9, 6.5, 5 and 4.1 kDa in addition to the PS I chlorophyll protein. N-terminal amino acid sequences of all these components were determined and compared with those of higher plants. Clearly, the 9.5 kDa component corresponds to the protein which carries the non-heme iron-sulfur centers A and B. This protein is so poorly visualized by staining that it has probably been overlooked in gel electrophoresis analyses. The 18, 14, 12 and 9 kDa components show appreciable homology with respective subunits of higher plant PS I. In contrast, the 6.5, 5 and 4.1 kDa components do not correspond to any known proteins except that the sequence of the 4.1 kDa component matches an unidentified open reading frame (ORF) 42 (liverwort) or ORF44 (tobacco) of chloroplast DNA.

Nucleotide sequences of cDNA clones encoding the entire precursor polypeptide for subunit VI and of the plastome-encoded gene for subunit VII of the photosystem I reaction center from spinach

Recombinant phage which encode the entire precursor polypeptide for subunit VI of the photosystem I reaction center have been selected from a lambda gt11 cDNA expression library made from polyadenylated RNA of spinach seedlings. The sequence predicts a precursor polypeptide of 144 amino acids (Mr = 15.3 kDa), a mature protein of 95 residues (Mr = 10.4 kDa) that lacks methionine, histidine and cysteine, and a transit peptide of 49 residues (Mr = 4.9 kDa). The corresponding gene(s) is (are) designated psaH. The gene for subunit VII, psaC, has been located in the small single-copy region of the spinach plastid chromosome using a synthetic oligonucleotide and a heterologous hybridization probe. It is part of a polycistronic transcription unit that is constitutively expressed and processed. Putative processing products include a monocistronic RNA for psaC. The polypeptide chain of 18 (deduced) amino acids is highly conserved and strikingly resembles bacterial-type ferredoxins. It harbours cysteine residues that appear to be involved in the ligation of the two 4Fe4S centres A and B in photosystem I. None of the two subunits appears to be membrane-spanning, and subunit VI, as subunit VII, is located at the reducing (stromal) side of the reaction center. All available information on the major subunits of photosystem I from spinach has been combined into a (revised) topographic model. Evidence that the innermost - plastome-encoded - core of photosystem I represents an old bacterial heritage in present day chloroplasts is discussed.

A cDNA clone encoding the precursor for a 10.2 kDa photosystem I polypeptide of barley

Two cDNA clones for the barley photosystem I polypeptide which migrates with an apparent molecular mass of 9.5 kDa on SDS-polyacrylamide gels have been isolated using antibodies and an oligonucleotide probe. The determined N-terminal amino acid sequence for the mature polypeptide confirms the identification of the clones. The 644 base-pair sequence of one of the clones contains one large open reading frame coding for a 14,882 Da precursor polypeptide. The molecular mass of the mature polypeptide is 10 193 Da. The hydropathy plot of the polypeptide shows one membranespanning region with a predicted alpha-helix secondary structure. The gene for the 9.5 kDa polypeptide has been designated PsaH.

Isolation and characterization of cDNA clones encoding the 17.9 and 8.1 kDa subunits of Photosystem I from Chlamydomonas reinhardtii

cDNA clones encoding two Photosystem I subunits of Chlamydomonas reinhardtii with apparent molecular masses of 18 and 11 kDa (thylakoid polypeptides 21 and 30; P21 and P30 respectively) were isolated using oligonucleotides, the sequences of which were deduced from the N-terminal amino acid sequences of the proteins. The cDNAs were sequenced and used to probe Southern and Northern blots. The Southern blot analysis indicates that both proteins are encoded by single-copy genes. The mRNA sizes of the two components are 1400 and 740 nucleotides, respectively. Comparison between the open reading frames of the cDNAs and the N-terminal amino acid sequences of the proteins indicates that the molecular masses of the mature proteins are 17.9 (P21) and 8.1 kDa (P30). Analysis of the deduced protein sequences predicts that both subunits are extrinsic membrane proteins with net positive charges. The amino acid sequences of the transit peptides suggest that P21 and P30 are routed towards the lumenal and stromal sides of the thylakoid membranes, respectively.

A photosystem II polypeptide is encoded by an open reading frame co-transcribed with genes for cytochrome b-559 in wheat chloroplast DNA

The N-terminal amino acid sequence of a 3.2 kDa photosystem II polypeptide is shown to be identical to that of a polypeptide encoded by an open reading frame of 38 codons (orf38) in wheat chloroplast DNA. Orf38 is located just downstream of the psbE and psbF genes for the polypeptides of cytochrome b-559. Analysis of the transcription of this region of chloroplast DNA shows that psbE, psbF and orf38 are co-transcribed to give a 1.1 kb polycistronic transcript which also contains another open reading frame of 40 codons. The orf38 and orf40 products are hydrophobic polypeptides which are both predicted to span the thylakoid membrane once. Orf38 and orf40 are highly conserved, and map to similar locations adjacent to psbE and psbF, in all organisms from which this region of DNA has been sequenced. We propose that orf38 is named psbL.

Nucleotide sequence of cDNA clones encoding the entire precursor polypeptides for subunits IV and V of the photosystem I reaction center from spinach

Using lambda gt11 expression cloning and immunoscreening, cDNA-containing recombinant phages for subunits IV and V of the photosystem I reaction center were isolated, sequenced and used to probe Northern blots of polyadenylated RNA prepared from spinach seedlings. The mRNA sizes for both components are approximately 1000 and 850 nucleotides, respectively. The 968 nucleotide cDNA sequence and derived amino acid sequence for subunit IV predict a single open reading frame of 231 amino acid residues (25.4 kDa). Comparison with a 13-residue N-terminal amino acid sequence determined for subunit IV suggests a mature protein of 17.3 kDa (154 residues) and a transit sequence of 77 amino acids (8.1 kDa). The corresponding data for subunit V are 677 bp (cDNA), 167 residues for the precursor protein (18.2 kDa), 98 residues for the mature polypeptide (10.8 kDa) and 69 residues for the transit peptide (7.4 kDa). Secondary structure predictions indicate that both proteins possess greatly different transit sequences and that none is membrane-spanning.

Cloning and sequencing of spinach cDNA clones encoding the 20 kDa PS I polypeptide

Apart from the 8 kDa subunit, which is of chloroplast origin, most of the small polypeptides of the PS I reaction center from higher plants are encoded in nuclear genes. We describe here the first nucleotide sequence of a nuclear component of this photosystem, the precursor of the 20 kDa protein. The deduced sequence of the large transit peptide (55-60 amino acids) is rich in serine/threonine residues and has a net positive charge, which are classical features of these precursors. The sequence itself is mainly hydrophilic, with no possibility of classical membrane-spanning alpha-helices; it exhibits an interesting stretch of five basic amino acids in close vicinity: Thr-Arg-Leu-Arg-Ser-Lys-Tyr-Lys-Ile-Lys-Tyr.

Localization and nucleotide sequence of the gene for the 8 kDa subunit of photosystem I in pea and wheat chloroplast DNA

The gene for the 8 kDa subunit of photosystem I has been located in the small single copy region of wheat chloroplast DNA by coupled transcription-translation of cloned fragments of DNA and by DNA sequence analysis. The pea gene for this subunit was located in pea chloroplast DNA by using the wheat gene as a probe. The location was confirmed by immunoprecipitation of the products of coupled transcription-translation of cloned DNA with antiserum raised against the small subunits of pea photosystem I and by DNA sequence analysis. The deduced amino acid sequences of the pea and wheat proteins are identical in seventy-six out of the eighty-one amino acid residues. There are nine conserved cysteine residues, eight of which are arranged in the primary structure in a similar way to those in bacterial ferredoxins containing two 4Fe-4S centres, suggesting that the polypeptide binds iron sulphur centres A and B of photosystem I.