A Photosystem-1 Reaction Center Complex Constituted Only by Two Subunits
A number of laboratories have reported on the preparation of Photosystem-1 reaction center complexes. They differ each other mainly in their subunit compositions. In a typical case, a complex consists of large subunits (50–70kD on SDS-PAGE) and several smaller ones (less than 20kD). It became clear that the large subunits are in fact two different polypeptides of 83 snd 82 kD’s, since two kinds of genes were found on a chloroplast DNA(l). The genes were designated as psaA and psaB (2). The large subunits have been implicated by numerous workers as the sites of the Photosystem-1 reaction center pigment, P700, a yet-to-be-identified primary electron acceptor(A0) and some other early electron acceptors (Vitamin Kl or A1, Component X or A2). One of the other smaller subunits, the 9kD polypeptide was isolated by several groups including us (3–5). From the N-terminal amino acid sequence, the gene for this subunit was identified on chloroplast DNAs and designated as either psaC(3,4) or frxA(5). Judging from the deduced primary structure, which was later confirmed by the sequencing of the peptide itself(6), the subunit is most likely an apoprotein for iron-sulfur clusters, Centers A and B. This has been confirmed recently by measuring EPR spectra of more intact preparations (7–9).
KeywordsMethyl Viologen Difference Spectrum Methane Sulfonic Acid Primary Electron Acceptor Reaction Center Preparation
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