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Abstract The RanBP2 zinc finger (Znf) domain is a prevalent domain that mediates protein interaction and RNA binding. In Arabidopsis, a clade of four RanBP2 Znf-containing proteins, named the Organelle Zinc (OZ) finger family, are known or predicted to be targeted to either the mitochondria or the plastids. Previously we reported that OZ1 is absolutely required for the editing of 14 sites in chloroplasts. We now have investigated the function of OZ2, whose null mutation is embryo lethal. We rescued the null mutant by expressing wild-type OZ2 under the control of the seed-specific ABSCISIC ACID-INSENSITIVE3 (ABI3) promoter. Rescued mutant plants exhibit severely delayed development and a distinctive morphological phenotype. Genetic and biochemical analyses demonstrated that OZ2 promotes the splicing of transcripts of several mitochondrial nad genes and rps3. The splicing defect of nad transcripts results in the destabilization of complex I, which in turn affects the respiratory ability of oz2 mutants, turning on the alternative respiratory pathway, and impacting the plant development. Protein-protein interaction assays demonstrated binding of OZ2 to several known mitochondrial splicing factors targeting the same splicing events. These findings extend the known functional repertoire of the RanBP2 zinc finger domain in nuclear splicing to include plant organelle splicing. 

The mitochondrial and chloroplastmRNAs of the majority of land plants are modified through cytidine to uridine (C‐to‐U)RNAediting. Previously, forward and reverse genetic screens demonstrated a requirement for pentatricopeptide repeat (PPR) proteins forRNAediting. Moreover, chloroplast editing factorsOZ1,RIP2,RIP9 andORRM1 were identified in co‐immunoprecipitation (co‐IP) experiments, albeit the minimal complex sufficient for editing activity was never deduced. The current study focuses on isolated, intact complexes that are capable of editing distinct sites. Peak editing activity for four sites was discovered in size‐exclusion chromatography (SEC) fractions ≥ 670 kDa, while fractions estimated to be approximately 413 kDa exhibited the greatest ability to convert a substrate containing the editing siterps14C80.RNAcontent peaked in the ≥ 670 kDa fraction. Treatment of active chloroplast extracts withRNase A abolished the relationship of editing activity with high‐MWfractions, suggesting a structuralRNAcomponent in native complexes. By immunoblotting,RIP9,OTP86,OZ1 andORRM1 were shown to be present in active gel filtration fractions, thoughOZ1 andORRM1 were mainly found in low‐MWinactive fractions. Active editing factor complexes were affinity‐purified using anti‐RIP9 antibodies, and orthologs to putativeArabidopsis thalianaRNAediting factorPPRproteins,RIP2,RIP9,RIP1,OZ1,ORRM1 andISE2 were identified via mass spectrometry. Western blots from co‐IP studies revealed the mutual association ofOTP86 andOZ1 with nativeRIP9 complexes. Thus,RIP9 complexes were discovered to be highly associated with C‐to‐URNAediting activity and other editing factors indicative of their critical role in vascular plant editosomes.