Gene expression in mitochondria is for the most controlled by nuclear encoded proteins that are from the cytosol into the organelle. Recent genetic and biochemical analysis have revealed that proteins of the PentatricoPeptide Repeat (PPR) family play preponderant and multifarious role in this process. During my thesis, I characterized the function of two novel Arabidopsis thaliana PPR protein called MTSF1 (for Mitochondrial Stability Factor 1) and PPR24. Each of these proteins is involved in the expression of a single mitochondrial gene encoding respiratory complex I subunit. I showed that the MTSF1 protein is essential for the stabilization of nad4 mRNA. The MTSF1 binding site was determined and was shown to correspond to the last twenty nucleotides of nad4 3’ UTR. We propose that, through an interaction with the extremity of nad4 transcript, the MTSF1 protein blocks the progression of 3’ to 5’ exoribonucleases and protects nad4 mRNA from degradation in vivo. PPR24 analysis indicated that this PPR protein is essential for nad7 mRNA translation. I observed that the NAD7 protein is not produced in ppr24 mutants and that this correlated with lack of ribosome loading of nad7 mature mRNA. I also showed that PPR24 binds to a short RNA fragment with high specificity located in the center of nad7 5’ leader. Structural predictions indicated that PPR24 binding site could correspond to the basis of a long stem-loop RNA structure just upstream of the AUG codon, which could prevent the accessibility to the nad7 translation codon. PPR24 binding could destabilize this stem-loop structure and permit a better access of the ribosome to the nad7 translation initiation codon. These findings shed light on the function and the mode of action of PPR proteins involved in mitochondrial gene expression in plants.
