Molecular Characterisation of two Putative Photosynthetic Proteins in Arabidopsis thaliana

Photosynthesis is the biological process, by which photosynthetic organisms convert light energy and carbon dioxide into organic compounds. In plants, photosynthesis takes place in the chloroplast, and this organelle evolved from the ancestor of cyanobacteria by endosymbiosis. The chloroplast proteome contains thousands of proteins, of which most are encoded by the nuclear genome. Transcriptome analysis reveals that nuclear photosynthetic genes and nuclear genes involved in chloroplast gene expression are co-regulated at the transcriptional level. To test this hypothesis, two co-regulated genes with so-far unknown biological function, named as Putative Photosynthetic Protein 1 (PPP1) and 3 (PPP3), were studied by reverse genetic approaches and biochemical analyses in Arabidopsis thaliana. The PPP3 knockout mutants and the PPP3-eGFP overexpressor in Arabidopsis have no distinct phenotype. PPP3 can form a small complex attached to thylakoid lamellae, and these interactions might be caused by two protein-protein interaction domains of PPP3 (TRP and PDZ). PPP1, also known as CSP41b, is a homolog of the spinach CSP41a, which is involved in chloroplast gene expression. BN-PAGE and RNA immunoprecipitation reveal that both Arabidopsis CSP41a and CSP41b are RNA-binding proteins, and prefer bind to chloroplast mRNAs rather than to rRNAs. The formation of CSP41-RNA complexes is regulated by the stromal redox state via post-translational modification. As indicated by RNA stability assay, CSP41b can stabilize chloroplast transcripts. Therefore, it can be concluded that the biological function of CSP41b is to protect RNA from degradation during the night. Such ready-to-use transcripts provide an economic way for plants rather than de novo RNA synthesis. Synthese.