Isolation and characterization of a cDNA clone encoding an 18-kDa hydrophobic photosystem I subunit (PSI-L) from barley (Hordeum vulgare L.)

Green plant photosystem I binds light-harvesting complex I on one side of the complex

We report a structural characterization by electron microscopy of green plant photosystem I solubilized by the mild detergent n-dodecyl-alpha-D-maltoside. It is shown by immunoblotting that the isolated complexes contain all photosystem I core proteins and all peripheral light-harvesting proteins. The electron microscopic analysis is based on a large data set of 14 000 negatively stained single-particle projections and reveals that most of the complexes are oval-shaped monomers. The monomers have a tendency to associate into artificial dimers, trimers, and tetramers in which the monomers are oppositely oriented. Classification of the dimeric complexes suggests that some of the monomers lack a part of the peripheral antenna. On the basis of a comparison with projections from trimeric photosystem I complexes from cyanobacteria, we conclude that light-harvesting complex I only binds to the core complex at the side of the photosystem I F/J subunits and does not cause structural hindrances for the type of trimerization observed in cyanobacterial photosystem I.

Photosystem I at 4 A resolution represents the first structural model of a joint photosynthetic reaction centre and core antenna system

The 4 A X-ray structure model of trimeric photosystem I of the cyanobacterium Synechococcus elongatus reveals 31 transmembrane, nine surface and three stromal alpha-helices per monomer, assigned to the 11 protein subunits: PsaA and PsaB are related by a pseudo two-fold axis normal to the membrane plane, along which the electron transfer pigments are arranged. 65 antenna chlorophyll a (Chl a) molecules separated by < or = 16 A form an oval, clustered net continuous with the electron transfer chain through the second and third Chl a pairs of the electron transfer system. This suggests a dual role for these Chl a both in excitation energy and electron transfer. The architecture of the protein core indicates quinone and iron-sulphur type reaction centres to have a common ancestor.

Characterization of psaI and psaL mutants of Synechococcus sp. strain PCC 7002: a new model for state transitions in cyanobacteria

The psaI and psaL genes were characterized from the cyanobacterium Synechococcus sp. strain PCC 7002. The gene organization was different from that reported for other cyanobacteria with psaI occurring upstream and being divergently transcribed from the psaL gene. Mutants lacking PsaI or PsaL were generated by interposon mutagenesis and characterized physiologically and biochemically. Mutant strains PR6307 (delta psaI), PR6308 (psaI-) and PR6309 (psaL-) had doubling times similar to that of the wild type under both high- and low-intensity white light, but all grew more slowly than the wild type in green light. Only monomeric photosystem I (PS I) complexes could be isolated from each mutant strain when Triton X-100 was used to solubilize thylakoid membranes; however, approximately 10% of the PS I complexes from the psaI mutants, but not the psaL mutant, could be isolated as trimers when n-dodecyl beta-D-maltoside was used. Compositional analyses of the mutant PS I complexes indicate that the presence of PsaL is required for trimer formation or stabilization and that PsaI plays a role in stabilizing the binding of both PsaL and PsaM to the PS I complex. Strain PR6309 (psaL-) was capable of performing a state 2 to state 1 transition approximately three times more rapidly than the wild type. Because the monomeric PS I complexes of this mutant should be capable of diffusing more rapidly than trimeric complexes, these data suggest that PS I complexes rather than phycobilisomes might move during state transitions. A "mobile-PS I" model for state transitions that incorporates these ideas is discussed.

A nuclear photosynthetic electron transport mutant of Arabidopsis thaliana with altered expression of the chloroplast petA gene

The nuclear photosynthetic mutant, hcf2, of Arabidopsis thaliana was isolated by screening M2 seedlings for abnormally-high chlorophyll fluorescence (hcf), indicative of a block in photosynthetic electron transport. Fluorescence induction kinetics, photosynthetic electron transport activity assays and immunoblotting revealed that all the complexes involved in photosynthetic electron transport were affected to some extent. The most striking effect of the mutation was on the relative steady state levels of the petA transcript (encoding the apoprotein of cytochrome f) which were more than five-times higher in the mutant plants than in their wild-type siblings.

Structure and expression of a nuclear gene for the PSI-D subunit of photosystem I in Nicotiana sylvestris

The PSI-D subunit is the ferredoxin-binding site of photosystem I, and is encoded by the nuclear gene psaD. We isolated a psaD genomic clone from Nicotiana sylvestris, by screening a genomic library with a psaD cDNA which we previously cloned from N. sylvestris (Yamamoto et al., Plant Mol Biol 17: 1251, 1991). Nucleotide sequence analysis revealed that this genomic clone contains a psaD gene, which does not correspond to the psaD cDNA, so we designated these genes psaDb and psaDa, respectively. The psaDb clone encodes a protein of 214 amino acids uninterrupted by introns. The N-terminal sequence determined for the N. sylvestris PSI-D protein encoded by psaDb begins at the 49th residue. The products of psaDa and psaDb share 82.7% and 79.5% identity at the amino acid and nucleotide levels, respectively. Genomic Southern analysis showed that two copies of psaD are present in the N. sylvestris genome. Ribonuclease protection assays and immunoblot analysis in N. sylvestris indicate that both genes are expressed in leaves, stems and flower buds, but neither is expressed in roots. During leaf development, the ratio of psaDb to psaDa mRNA increases from 0.12 in leaf buds to 0.36 in mature leaves. The relative abundance of the corresponding proteins decreased over the same developmental period. These results indicate that differential regulation mechanisms control psaDa and psaDb expression at both the mRNA and protein levels during leaf development.

Isolation and characterization of cDNA clones encoding a 18.8 kDa polypeptide, the product of the gene psaL, associated with photosystem I reaction center from spinach

Several cDNA clones encoding subunit XI of photosystem I reaction center (PSI-L) have been isolated from two lambda gt11 expression libraries based on polyadenylated RNA of spinach seedlings illuminated for 4 and 16 h, respectively. The precursor polypeptide made from these recombinant DNAs in vitro can be efficiently imported into isolated spinach chloroplasts. It is correctly processed to the size of the authentic polypeptide and integrates into the photosystem I assembly. The 834 nucleotide sequence of the longest cDNA insert encodes a precursor polypeptide of 24 kDa (216 residues) and a mature protein of probably 18.8 kDa (169 residues). Hydropathy analysis suggests that the polypeptide contains two transmembrane segments. The protein appears to originate in a single-copy gene in spinach and to be decoded from RNA species of ca. 900 bases.

The primary structure of a cDNA for PsaN, encoding an extrinsic lumenal polypeptide of barley photosystem I

A cDNA clone encoding a 15,501 Da photosystem I (PSI) subunit of barley was isolated using an oligonucleotide based on the NH2-terminal amino acid sequence of the isolated protein. The polypeptide, which migrates with an apparent molecular mass of 9.5 kDa on denaturing SDS-PAGE, has been designated PSI-N, and the corresponding gene is PsaN. Analysis of the deduced protein sequence indicates a mature protein of 85 amino acid residues and a molecular mass of 9818 Da. PSI-N is a hydrophilic, extrinsic protein with no predicted membrane-spanning regions. The transit peptide of 60 residues (5683 Da) contains a predicted hydrophobic alpha-helix, suggesting that the protein is routed into the thylakoid lumen. Thus, PSI-N is the second known lumenal protein component associated with PSI, together with PSI-F.