A PORR domain protein required forrpl2andccmFC intron splicing and for the biogenesis ofc-type cytochromes in Arabidopsis mitochondria

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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:rpl2andccmFC. 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 inwtf9mutants as a means to functionally characterize the mitochondrialrpl2andccmFC genes. Loss ofccmFC expression correlates with the loss of cytochromescandc1, confirming a role forccmFC in cytochrome biogenesis. By contrast, our results strongly suggest that splicing is not essential for the function of the mitochondrialrpl2gene, and imply that the Rpl2 fragment encoded byrpl2exon 1 functions in concert with a nuclear gene product that provides the remainder of this essential ribosomal proteinin trans.

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