A Plant Mitochondrial Sequence Transcribed in Transgenic Tobacco Chloroplasts Is Not Edited

Abstract

RNA editing occurs in two higher-plant organelles, chloroplasts and mitochondria. Because chloroplasts and mitochondria exhibit some similarity in editing site selection, we investigated whether mitochondrial RNA sequences could be edited in chloroplasts. We produced transgenic tobacco plants that contained chimeric genes in which the second exon of a Petunia hybrida mitochondrial coxII gene was under the control of chloroplast gene regulatory sequences. coxII transcripts accumulated to low or high levels in transgenic chloroplasts containing chimeric genes with the plastid ribosomal protein gene rps16 or the rRNA operon promoter, respectively. Exon 2 of coxII was chosen because it carries seven editing sites and is edited in petunia mitochondria even when located in an abnormal context in an aberrant recombined gene. When editing of the coxII transcripts in transgenic chloroplasts was examined, no RNA editing at any of the usual sites was detected, nor was there any novel editing at any other sites. These results indicate that the RNA editing mechanisms of chloroplasts and mitochondria are not identical but must have at least some organelle-specific components.The mRNAs of both plant mitochondria and chloroplasts have been shown to be subject to RNA editing (8,11,13,14). T's are found in cDNAs of both chloroplasts and mitochondria where C's are located in genomic DNA. Conversion of cytidine to uridine has been demonstrated to occur in plant mitochondria (23). Most mitochondrial mRNAs have been shown to contain one or more RNA editing sites (12,24). In contrast, many fewer editing events have been found in chloroplast mRNAs (15). Nevertheless, some sequence similarities exist between chloroplast and mitochondrial RNA editing sites. Editing occurs in transcripts of the chloroplast ndhA and ndhB and mitochondrial nad1 and nad2 genes at sites which are homologous at the protein level (17,18). Both editing systems exhibit a biased codon transition frequency in which serine and proline codons are more likely to be targets of editing than other C-containing codons. C's in the third position of the codon or C's that follow G residues are the least likely to be edited (7). Editing can occur before transcript splicing in both organelles. However, unspliced transcripts in both mitochondria and chloroplasts are less likely to be edited in all potential editing sites than spliced transcripts (10, 27, 31), suggesting that editing in both organelles is a posttranscriptional process.We wished to test whether mitochondrial sequences could be targets for the editing machinery of chloroplasts. To that end, we cloned the coding portion of the second exon of petunia coxII-2 into two different tobacco plastid expression cassettes and introduced the chimeric genes into the tobacco plastid genome. The coxII-2 second exon contains seven sites that are edited in petunia mitochondria. Although coxII transcripts from both chimeric genes were detected in chloroplasts of transgenic tobacco plants, we observed no RNA editing. Evidently, the ...

Fate of a Larch Unedited tRNA Precursor Expressed in Potato Mitochondria

“…33). For instance, when the second exon of petunia mitochondrial coxII-2 gene was transcribed in transgenic tobacco chloroplasts, no RNA editing was observed, demonstrating the presence of editing components that are specific of each organelle (34). Furthermore, transcripts of mitochondrial Sorghum bicolor atp6-1 gene expressed in organello upon introduction via electroporation into isolated maize mitochondria, were not edited (35).…”

cis Recognition Elements in Plant Mitochondrion RNA Editing

“…It is essential in plant mitochondrion gene expression processes such as the maturation step of organellar transcripts (26,27) or the synthesis of functional proteins, since the nucleotide conversions usually alter the coding properties of the mRNA (1). The editing systems in higher plant organelles, mitochondria, and chloroplasts share many similar features, but promiscuous chloroplast sequences are not edited in mitochondria (39); conversely, a mitochondrial sequence carrying an editing site does not sustain editing when transcribed into chloroplasts (35). These results indicate that editing recognition signals are specific to each organelle.…”

Cross-Competition in Transgenic Chloroplasts Expressing Single Editing Sites Reveals SharedcisElements

“…A number of nucleus-encoded proteins are known to be targeted to both chloroplasts and mitochondria. Arguing against sharing of factors is an experiment in which mitochondrial sequences were introduced into transgenic chloroplasts (27). None of the seven sites in exon 2 of coxII were edited when introduced into chloroplasts, even though one of the sites (Fig.…”