Photosystem I Function and Assembly in Tobacco Chloroplast Mutants

Abstract

Photosystem I is a thylakoid membrane complex that functions as a plastocyaninferredoxin oxidoreductase (1). Light energy is captured by antenna pigments and rapidly transferred to the reaction center chlorophylls, P₇₀₀ and electron transfer is initiated. Electrons pass through the intermediary electron acceptors A₀, A₁, F_X, F_A and F_B to ferredoxin. Photosystem I consists of at least thirteen individual polypeptides named PsaA-PsaN (depending on the organism). All electron transfer intermediates between plastocyanin and ferredoxin are coordinated by three chloroplast encoded proteins, PsaA, PsaB and PsaC. The chloroplast genome also encodes Psa1, PsaJ, Ycf3 and Ycf4 that are either associated with photosystem I or required for its stable accumulation. The function of these open reading frames in photosystem I biogenesis is not well established. Deletion of PsaI and PsaJ in cyanobacteria (2,3) and Chlamydomonas (4) has no significant impact on photosystem I phenotype. Biochemical analysis suggests PsaI and PsaJ may be required for structural organization of PsaL and PsaF, respectively (2, 3). Ycf3 and Ycf4 have not yet been identified as structural components of photosystem I, but are required for its normal accumulation (5, 6). As part of a wider study of the function of unidentified chloroplast open reading frames in tobacco we are generating deletion and site-directed mutations of the aforementioned genes that should improve our understanding of the role of the chloroplast encoded proteins in photosystem I biogenesis.

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