Faithful editing of a tomato-specific mRNA editing site in transgenic tobacco chloroplasts

RNA editing sites and their site-specific trans-acting recognition factors are thought to have coevolved. Hence, evolutionary loss of an editing site by a genomic mutation is normally followed by the loss of the specific recognition factor for this site, due to the absence of selective pressure for its maintenance. Here, we have tested this scenario for the only tomato-specific plastid RNA editing site. A single C-to-U editing site in the tomato rps12 gene is absent from the tobacco and nightshade plastid genomes, where the presence of a genomic T nucleotide obviates the need for editing of the rps12 mRNA. We have introduced the tomato editing site into the tobacco rps12 gene by plastid transformation and find that, surprisingly, this heterologous site is efficiently edited in the transplastomic plants. This suggests that the trans-acting recognition factor for the rps12 editing site has been maintained, presumably because it serves another function in tobacco plastids. Bioinformatics analyses identified an editing site in the rpoB gene of tobacco and tomato whose sequence context exhibits striking similarity to that of the tomato rps12 editing site. This may suggest that requirement for rpoB editing resulted in maintenance of the rps12 editing activity or, alternatively, the pre-existing rpoB editing activity facilitated the evolution of a novel editing site in rps12.     

Pervasive RNA Editing Among Hornwort rbcL Transcripts Except Leiosporoceros

RNA editing affecting chloroplast and mitochondrial genomes has been identified in all major clades of land plants. The frequency of edited sites varies greatly between lineages but hornworts represent an extreme in propensity for editing in both their chloroplast and mitochondrial genomes. cDNA sequences from seven taxonomically diverse hornwort rbcL sequences combined with a survey of 13 additional DNA sequences for potential edited sites demonstrate the presence of 62 edited sites and predict a minimum of 10 additional sites. These 72 total edited sites represent 43 C-to-U and 28 U-to-C nucleotide conversions, with 1 site exhibiting editing in both directions. With one exception, all taxa are heavily edited, with each having from 20 to 34 edited sites. However, a single sample, Leiosporoceros, is shown to lack edited sites. Phylogenetic reconstruction of hornworts results in ambiguous resolution of Leiosporoceros depending on whether edited sites are maintained or eliminated from the analyses. Depending on the inferred relationship of Leiosporoceros to the hornworts, at least two explanations for the origin and maintenance of pervasive editing in hornworts are possible. The absence of edited sites in Leiosporoceros could represent either the absence or a low level of editing ability in the common ancestor of hornworts, as represented by Leiosporoceros, or the loss of editing sites in this lineage after the primary diversification events in the group.     

Editing of the grapevine mitochondrial cytochrome b mRNA and molecular modeling of the protein

Cytochrome b (COB), the central catalytic subunit of ubiquinol cytochrome c reductase, is a component of the transmembrane electron transfer chain that generates proton motive force. Some plant COB mRNAs are processed by RNA editing, which changes the gene coding sequence. This report presents the sequences of the grapevine (Vitis vinifera L.) mitochondrial gene for apocytochrome b (cob), the edited mRNA and the deduced protein. Grapevine COB is 393 amino acids long and is 98% identical to homologs in rapeseed, Arabidopsis thaliana and Oenothera sp. Twenty-one C-U editing sites were identified in the grapevine cob mRNA, resulting in 20 amino acid changes. These changes increase the overall hydrophobicity of the protein and result in a more conserved protein. Molecular modeling of grapevine COB shows that residues changed by RNA editing fit the secondary structure characteristic of an integral membrane protein. This is the first complete mitochondrial gene reported for grapevine. Novel RNA editing sites were identified in grapevine cob, which have not been previously reported for other plants.