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

Abstract

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 PSI 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-berne 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.

Amino acid sequence of photosystem I subunit IV from the cyanobacterium Synechocystis PCC 6803

“…The calculated molecular mass is 8000, in good agreement with the electrophoretic migration of this protein (about 9 kDa); this was not the case for all other subunit IVs so far described and will be discussed later. The full sequence fits well with 'partial sequences previously published for other cyanobacteria [15,16]. The hydrophilicity profile [28] is presented in fig.3, in comparison with the corresponding profile of the barley polypeptide.…”

Polypeptide composition of higher plant photosystem I complex

“…2) clearly corresponded to the Nterminal sequences deduced from ORF42/44 of plant chloroplast DNA [27][28][29]. The Synechococcus 4.1 kDa protein, whose N-terminus was also blocked [17], was largely homologous but somewhat more divergent in its N-terminal region. Based on the correspondence of Nterminal sequence of the Synechococcus 4.1 kDa protein to the DNA sequence of plant ORF42/44, we tentatively designated this ORF psaJ [30].…”

“…In addition to these components, we recently found three new low-molecular-mass components of 4.1 kDa, 5 kDa and 6.5 kDa in cyanobacterial PSI complex and reported their partial amino acid sequences [17]. Interestingly, the sequence of this cyanobacterial 4.1 kDa component corresponded to ORF42/44 of higher plant chloroplast DNA, although its product had not yet been found in plants.…”

“…'Native' PSI complex which additionally retains LHCI can be isolated from green plants by a milder detergent treatment, while cyanobacterial PSI has no LHCI-like antenna. Until recently the subunit proteins found in these various types of PSI complexes from both green plants and cyanobacteria have been classified in three categories as follows: (i) two chlorophyll a-binding subunits of about 80 kDa, which carry P700, acceptors A0 and A1, and Fe-S center X, (ii) 5-6 core subunits ranging from 8 to 22 kDa which include Fe-S center A/B protein, plastocyanin-docking protein and ferredoxin-docking protein, (iii) 4-5 LHCI subunits of Abbreviations: CPI, chlorophyll-protein complex 1 of photosystem I reaction center; LHCI, light-harvesting chlorophyll complex associated with photosystem I; PSI, photosystem I; SDS-PAGE, sodium dodecylsulfate-polyacrylamide gel electrophoresis green plants ranging from 15 to 25 kDa which bind antenna chlorophyll a and b. Sequencing of these proteins and/or their corresponding genes have registered two genes, psaA and psaB, for the P700-carrying proteins [2,3], 6 genes, psaC to psaH, for the core subunits [4][5][6][7][8][9][10][11][12][13][14][15][16][17] and at least three types of cab genes for the LHCI subunits [18][19][20]. In green plants, psaA, psaB and psaC are encoded by chloroplast genome while the others are encoded by nuclear genome.…”

“…In green plants, psaA, psaB and psaC are encoded by chloroplast genome while the others are encoded by nuclear genome. Sequence comparison indicates that the two P700-carrying proteins and most of the core subunits are more or less conserved between cyanobacteria and green plants [3,13,17], although LHCI is absent in cyanobacteria.…”