Is RNA editing implicated in group II intron survival in the angiosperm mitochondrial genome?

摘要

Introns may be considered as optional because they are removed from mRNA molecules, butintrons are fairly preserved for unknown reasons. Previously, the mitochondrial rps3 gene ofsugar beet (Beta vulgaris L., Caryophyllales) was shown to represent a unique example of anintron loss. We have determined the distribution of the rps3 intron in 19 Caryophyllaleanspecies. The intron was absent from the Amaranthaceae and the Achatocarpaceae. In theCaryophyllaceae, Dianthus japonicus rps3 was pseudogenized but the intronic sequence wasretained. Intact intron-bearing rps3 copies were cloned from Portulaca grandiflora andMyrtillocactus geometrizans, members of the sister clade of theAmaranthaceae-Achatocarpaceae-Caryophyllaceae clade. Most of the C-to-U RNA-editingsites in Portulaca and Myrtillocactus rps3 transcripts were homologous in the two species aswell as in the sugar beet rps3, which, unlike other 12 rps3 transcripts, lacks editing in the exonicregions around the intron. Provided that the loss of editing preceded the loss of rps3 intron, itappears conceivable that a requirement for editing could have prevented the loss of group-IIintrons retained in angiosperm mitochondrial genomes. This interpretation is an alternative tothe conventional one that views the loss of editing as a mere trace of RNA-mediated geneconversion.