RNA PROCESSING FACTOR 3 is crucial for the accumulation of mature ccmC transcripts in mitochondria of Arabidopsis thaliana accession Columbia

RNA PROCESSING FACTORS 1 and 2 are pentatricopeptide repeat (PPR) proteins involved in 5' processing of different mitochondrial mRNAs in Arabidopsis thaliana . Both factors are highly similar to RESTORERS OF FERTILITY (RF), which are part of cytoplasmic male sterility/restoration systems in various plant species. These findings suggest a predominant role of RF-like PPR proteins in posttranscriptional 5' processing. To further explore the functions of this group of proteins we examined a number of T-DNA lines carrying insertions in the corresponding PPR genes. This screening identified a nearly complete absence of mature ccmC transcripts in an At1g62930 T-DNA insertion line, a phenotype that could be restored by the introduction of the intact At1g62930 gene into the mutant. The insertion in this nuclear gene, which we now call RNA PROCESSING FACTOR 3, also leads to a severe reduction of the CcmC protein in mitochondria. The analysis of C24/ rpf3-1 F 2 hybrids lacking functional RPF3 genes revealed that this gene has less influence on the generation of the mature ccmC 5' transcript end derived from a distinct ccmC 5’ upstream configuration found in mitochondrial DNAs from C24 and other accessions. These data show that a particular function of an RF-like protein is required only in connection with a distinct mtDNA configuration. Our new results further substantiate the fundamental role of RF-like PPR proteins in the posttranscriptional generation of plant mitochondrial 5’ transcript termini. control could be amplified from two F 2 plants. This result was confirmed by a northern blot hybridization, which confirmed identical sizes of mature ccmC mRNAs in these F 2 and C24 wild-type plants. But the northern analysis also revealed a reduced level of the mature ccmC transcripts in the F 2 hybrids. These results demonstrate that the function of the RPF3 gene is per se not required for the generation of substantial amounts of the mature C24-type ccmC mRNA in these F 2 hybrids and probably also in C24. These observations suggest that a distinct mtDNA configuration, i.e. in the ccmC upstream region, might be associated with a particular version of the RPF3 reading frame. Thus we examined the RPF3 coding