A PORR domain protein required for rpl2 and ccmF _C intron splicing and for the biogenesis of c-type cytochromes in Arabidopsis mitochondria

Angiosperm mitochondria encode approximately 20 group II introns, which interrupt genes involved in the biogenesis and function of the respiratory chain. Nucleus-encoded splicing factors have been identified for approximately half of these introns. The splicing factors derive from several protein families defined by atypical RNA binding domains that function primarily in organelles. We show here that the Arabidopsis protein WTF9 is essential for the splicing of group II introns in two mitochondrial genes for which splicing factors had not previously been identified: rpl2 and ccmF _C. WTF9 harbors a recently recognized RNA binding domain, the PORR domain, which was originally characterized in the chloroplast splicing factor WTF1. These findings show that the PORR domain family also functions in plant mitochondria, and highlight the parallels between the machineries for group II intron splicing in plant mitochondria and chloroplasts. In addition, we used the splicing defects in wtf9 mutants as a means to functionally characterize the mitochondrial rpl2 and ccmF _C genes. Loss of ccmF _C expression correlates with the loss of cytochromes c and c ₁, confirming a role for ccmF _C in cytochrome biogenesis. By contrast, our results strongly suggest that splicing is not essential for the function of the mitochondrial rpl2 gene, and imply that the Rpl2 fragment encoded by rpl2 exon 1 functions in concert with a nuclear gene product that provides the remainder of this essential ribosomal protein in trans.