PPR596 Is Required for nad2 mRNA Splicing and Complex I Biogenesis in Mitochondria of Arabidopsis thaliana

Several genes in the mitochondria of angiosperms are interrupted by introns, and their posttranscriptional excision involves numerous nucleus-encoded auxiliary factors. Most of these factors are of eukaryotic origin, among them members of the pentatricopeptide-repeat (PPR) family of RNA-binding proteins. This family divides into the PLS and P classes, with PLS-class proteins typically participating in C-to-U mRNA editing and P-class members contributing to transcript stabilization and intron splicing. The P-class protein PPR596 was previously described to be involved in mitochondrial RNA editing, with the ppr596 mutant showing moderately elevated editing of a specific, partially edited site within the rps3 mRNA. PPR596 disruption led to a substantial delay in plant development. Because the moderate change in RNA editing in the ppr596 mutant is unlikely to be the cause of its severe developmental retardation, we re-investigated mitochondrial gene expression and found that PPR596 is specifically required for the efficient excision of the third intron from the nad2 pre-mRNA. Insufficient splicing of this intron in ppr596 impairs respiratory-chain complex I biogenesis at the step of the insertion of the Nad2 subunit, thus perturbing plant development.