SUMMARYThe 5′ ends of many mitochondrial transcripts are generated post-transcriptionally. Recently, we identified three RNA PROCESSING FACTORs required for 5′ end maturation of different mitochondrial mRNAs in Arabidopsis thaliana. All of these factors are pentatricopeptide repeat proteins (PPRPs), highly similar to RESTORERs OF FERTILTY (RF), that rescue male fertility in cytoplasmic male-sterile lines from different species. Therefore, we suggested a general role of these RF-like PPRPs in mitochondrial 5′ processing. We now identified RNA PROCESSING FACTOR 5, a PPRP not classified as an RF-like protein, required for the efficient 5′ maturation of the nad6 and atp9 mRNAs as well as 26S rRNA. The precursor molecules of these RNAs share conserved sequence elements, approximately ranging from positions À50 to +9 relative to mature 5′ mRNA termini, suggesting these sequences to be at least part of the cis elements required for processing. The knockout of RPF5 has only a moderate influence on 5′ processing of atp9 mRNA, whereas the generation of the mature nad6 mRNA and 26S rRNA is almost completely abolished in the mutant. The latter leads to a 50% decrease of total 26S rRNA species, resulting in an imbalance between the large rRNA and 18S rRNA. Despite these severe changes in RNA levels and in the proportion between the 26S and 18S rRNAs, mitochondrial protein levels appear to be unaltered in the mutant, whereas seed germination capacity is markedly reduced.