Mutation of the pentatricopeptide repeat-SMR protein SVR7 impairs accumulation and translation of chloroplast ATP synthase subunits in Arabidopsis thaliana

RNA processing, RNA editing, RNA splicing and translational activation of RNAs are essential post-transcriptional steps in chloroplast gene expression. Typically, the factors mediating those processes are nuclear encoded and post-translationally imported into the chloroplasts. In land plants, members of the large pentatricopeptide repeat (PPR) protein family are required for individual steps in chloroplast RNA processing. Interestingly, a subgroup of PPR proteins carries a C-terminal small MutS related (SMR) domain. Here we analyzed the consequences of mutations in the SVR7 gene, which encodes a PPR-SMR protein, in Arabidopsis thaliana. We demonstrate that SVR7 mutations lead to a specific reduction in chloroplast ATP synthase levels. Furthermore, we found aberrant transcript patterns for ATP synthase coding mRNAs in svr7 mutants. Finally, a reduced ribosome association of atpB/E and rbcL mRNAs in svr7 mutants suggests the involvement of the PPR-SMR protein SVR7 in translational activation of these mRNAs. We describe that the function of SVR7 in translation has expanded relative to its maize ortholog ATP4. The results provide evidence for a relaxed functional conservation of this PPR-SMR protein in eudicotyledonous and monocotyledonous plants, thus adding to the knowledge about the function and evolution of PPR-SMR proteins.