Lack of conservation of editing sites in mRNAs that encode subunits of theNAD(P)H dehydrogenase complex in plastids and mitochondria of Arabidopsis thaliana

RNA editing in the plastids and mitochondria of higher plants involves C to U conversion of specific nucleotides in the mRNA. This leads to the synthe sis of proteins that are different from those predicted by the DNA sequence . Editing appears to have arisen at about the same time in both plastids an d mitochondria, suggesting a common evolutionary origin. The problem we add ress here is whether or not there has been co-evolution of the editing syst ems in the two organelles. Our test system was editing of the Arabidopsis t haliana mRNAs for ndhB and nad2, and for ndhD and nad4. which encode homolo gous subunits of the plastid and mitochondria) NAD(P)H dehydrogenases, resp ectively. The editing sites in the Arabidopsis nad2 and nad4 mRNAs have pre viously been determined and we report here 19 editing sites in eight mRNAs in Arabidopsis plastids. Out of these, eight sites are localized in the ndh B mRNA. In its mitochondrial counterpart, nad 2, 31 editing sites are prese nt, none of which are shared with the ndhB gene. The Arabidopsis ndhD mRNA is edited at four positions, only one of which is shared by its mitochondri al homologue, nad4, which contains 32 editing sites. These findings suggest that, although editing in the two organelles may have derived from a singl e system, there is no significant conservation of editing sites in cognate mRNAs in plastids and mitochondria.