Genetic Dissection of Photosystem I Assembly and Turnover in Eukaryotes
The photosystem I (PS I) complex of plants and algae is a large multisubunit protein complex consisting of nucleusand chloroplast-encoded subunits. Besides its redox cofactors P700, A0, A1, FX, FA, and FB, the PS I complex also contains a core antenna consisting of 90 chlorophyll and 22 carotenoid molecules. An extensive forward and reverse genetics approach in Chlamydomonas and Arabidopsis has provided important insights into the mechanisms of synthesis of the subunits and their assembly into a functional complex. The picture which emerges from these studies is that a surprisingly large number of nucleus-encoded factors are involved in several post-transcriptional steps of expression of the two large chloroplast-encoded reaction center subunits PsaA and PsaB. Thus 14 factors are required for the maturation of the psaA mRNA through two trans-splicing reactions, two factors are required for the translation of the psaB mRNA, and one is required for psaB mRNA stability in Chlamydomonas. Several additional factors, including Ycf3 and Ycf4, are specifically required for the assembly of the PS I complex. With its three [4Fe–4S] clusters, PS I constitutes an important sink for iron. Its specific and early loss following iron deprivation is a phylogenetically conserved process and is preceded by a remodeling of the PS I antenna. Coupled genetic and biochemical approaches have revealed a new link between PS I assembly and the chlorophyll biosynthetic pathway through the identification of Crd1, a di-iron enzyme possibly involved in the aerobic oxidative cyclase reaction of chlorophyll synthesis that is essential for PS I and LHCI accumulation under copper deprivation.
KeywordsThylakoid Membrane Redox Cofactor Chloroplast Gene Expression Reaction Center Subunit psaA mRNA
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