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To gain insight into the biogenesis of photosystem II (PSII) and to identify auxiliary factors required for this process, we characterized the mutant hcf173 of Arabidopsis thaliana. The mutant shows a high chlorophyll fluorescence phenotype (hcf) and is severely affected in the accumulation of PSII subunits. In vivo labeling experiments revealed a drastically decreased synthesis of the reaction center protein D1. Polysome association experiments suggest that this is primarily caused by reduced translation initiation of the corresponding psbA mRNA. Comparison of mRNA steady state levels indicated that the psbA mRNA is significantly reduced in hcf173. Furthermore, the determination of the psbA mRNA half-life revealed an impaired RNA stability. The HCF173 gene was identified by map-based cloning, and its identity was confirmed by complementation of the hcf phenotype. HCF173 encodes a protein with weak similarities to the superfamily of the short-chain dehydrogenases/reductases. The protein HCF173 is localized in the chloroplast, where it is mainly associated with the membrane system and is part of a higher molecular weight complex. Affinity chromatography of an HCF173 fusion protein uncovered the psbA mRNA as a component of this complex.
INTRODUCTION
Photosystem II (PSII) is one of the three multisubunit protein complexes of the photosynthetic electron transport chain. It is localized in the thylakoid membranes of photosynthetically active organisms and functions as a water-plastoquinone oxidoreductase. Like other complexes of the thylakoid membrane, PSII is a genetic mosaic consisting of plastid- and nuclear-encoded subunits (Wollman et al., 1999); therefore, a coordination of gene expression in the plastid and the cytosol is required (GoldschmidtClermont, 1998).
The core complex of the PSII reaction center consists of the proteins D1 and D2, which bind all important cofactors needed for the primary charge separation (i.e., the P680 chlorophyll a, pheophytin, and plastoquinone). The 32-kD D1 protein is encoded by the psbA gene of the chloroplast genome. Regulation of D1 protein synthesis is important for the correct biogenesis of PSII during chloroplast development and for the maintenance of a functional photosystem, since D1 exhibits a high turnover due to damage conferred by light. The accumulation of the D1 protein depends on environmental signals, such as light and the developmental stage of the plant (Klein et al., 1988; Gamble and Mullet, 1989; Klein and Mullet, 1990; Klaff...