Molecular And Functional Characterization Of AtCTPA1 Gene In Arabidopsis Thaliana

Photosystem II (PSII), which catalyzes light-dependent water oxidation and plastoquinone reduction in chloroplasts, is a large pigment-protein complex embedded in the thylakoid membrane. PSII consists of more than 25 intrinsic and extrinsic proteins. The PSII reaction center complex is composed of the D1 and D2 proteins, the a- and 6-subunits of cytochrome b559, and the PsbI protein. The D1 and D2 heterodimer binds all the essential redox components of PSII required to transfer electrons from the manganese cluster of the water-oxidizing complex to the plastoquinone pool.Light-induced damage is inevitable for PSII due to its normal function in energy convertion and primary charge seperation, with the reaction center D1 protein being the main target among PSII subunits. Oxygenic organisms have evolved an efficient repair mechanism to maintein PSII in a functional state. Damaged D1 proteins are degraded by proteases in chloroplast and subsequently replaced with newly synthesized copies. In higher plants the reaction center D1 protein is synthesized as a precursor with a carboxyl-terminal extension containing 9 anmino acid residues. This extension must be removed by a specific carboxyl-terminal processing protease of D1 (CtpA) for the assembly of a functional PSII. This is important for both PSII biogenesis and efficient PSII repair cycle. In this study we identified and characterized a T-DNA insertion mutant of a CtpA homologue (AtCTPA1) in Arabidopsis by employing a reverse genetics approach. Our results demostrated that the AtCTPA1 gene encoded a soluble lumenal protein of～47 kDa in Arabidopsis. The T-DNA insertion abolished the expression of AtCTPA1 in atctpa1 mutants. Disruption of AtCTPA1 did not affect the fitness of atctpa1 mutants grown under optimal light. Mutant plants exhibited normal growth rate and pigmentation, and have a similar oxygen-evolving activity as WT plants. The chlorophyll fluorescence characteristics and PSII activity (Fv/Fm) remains unchanged in mutant plants. Contents of major pigment-protein complexes in thylakoid membrane and their subunit composition is comparable to WT in atctpa1 mutant. Nevertheless, the gene AtCTPA1 was important for plant survival under high light conditions. Mutants exhibited increased susceptibility to high irradiance. Compared with the WT plants, PSII activity (Fv/Fm) of atctpa1 mutants declined faster during photoinhibiton but restored more slowly upon recovery. Lincomycin, a inhibitor for chloroplast protein synthesis, cause no difference in the decrease rate of PSII activity between WT and atctpa1 mutant plants. The contents of D1 dropped more quickly in atctpa1 than in WT plants during photoinhibition. A comparable decrease rate of D1 was observed between atctpa1 and WT following a priror treatment with licomycin. Taken together, these results demonstrated that AtCTPA1 gene was involved in the efficient repair of PSII and turnover of PSII reaction center protein D1 under high light.