Structural studies on cyanobacterial photosystem I

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

“…Note that the 9.5 kDa band is very weak and diffuse, in agreement with the known properties of higher plant iron-sulfur proteins [28]. Since the 6.5, 5 and 4.1 kDa proteins are not resolved by conventional SDS-PAGE (not shown), this separation profile would be basically the same as those reported for other cyanobacterial PSI complexes [3][4][5][6]. For example, Synechococcus PCC 6301 PS I complex has been reported to contain 18, 17.7, 16 and 10 kDa with a molar ratio of 0.7: 1.0:0.5:1.6 [3], which seem to correspond to our 18, 14, 12 and 9 kDa components with a compatible ratio.…”

“…However, direct identification of the protein was difficult probably because this protein is so poorly stained on the gel: the iron-sulfur protein has been overlooked on the gel in spite of its abundance in the PSI complex. On the other hand, four low-molecular-mass proteins have been typically resolved in cyanobacteria by conventional SDS-PAGE, and their characterization and correspondence have been sporadically reported [3][4][5][6]21,22]. In this study, we resolved eight proteins and showed more clearly the correspondence between the 18, 14, 12 and 9 kDa proteins of S.…”

“…The determined N-terminal sequence of the 18 kDa protein ( fig.2A) shows high homology to those of the 17.7 kDa protein of Synechococcus PCC 6301 [4] and the 16kDa protein of Synechocystis PCC 6803 [21]. It is also homologous to the internal sequence of subunit II (20-22 kDa) of spinach [17] and tomato [15].…”

“…The other core components of higher plants so far sequenced are all encoded by nuclear genome and their sequences vary considerably among plant species [15][16][17][18][19][20]. Some such core components from cyanobacteria have been also sequenced, but only one component has been proven to correspond to a higher plant homologue (subunit II or psaD product) [4,21,22]. Our l~esent knowledge about the correspondence of the other low-molecular-mass core components remains very limited.…”

“…In higher plants, the PSI complex consists of three groups of protein components: (i) two CP I apoproteins (psaA and psaB gene products) bearing PT00, chlorophylls and a non-heine iron-sulfur center X; (ii) several components of 8 to 22 kDa which belong to the PS I core complex; (iii) several LHC I components of 20-25 kDa [1,2]. Similarly, cyanobacterial PSI complex has been reported to contain four components of 8 to Abbreviations: CP I, photosystem I chlorophyll-protein; EPR, electron paramagnetic resonance; LHC I, light-harvesting chlorophyll-protein complex associated with photosystem I; ORF, open reading frame; PS 1 and II, photosystem I and II, respectively; PVDF, polyvinylidene difluoride; SDS-PAGE, SDS-polyacrylamide gel electrophoresis 20 kDa and the two CP I apoproteins [3][4][5][6]. Since the cyanobacterial complex does not carry any LHC I-type components, these low-molecularmass components might have structural or functional roles similar to the core components of higher plants.…”