Cyanophora paradoxa mitochondrial tRNAs play a double game

Present-day mitochondria derive from a single endosymbiosis of an α-proteobacterium into a proto-eukaryotic cell. Since this monophyletic event, mitochondria have evolved considerably, and unique traits have been independently acquired in the different eukaryotic kingdoms. Mitochondrial genome expression and RNA metabolism have diverged greatly. Here, Cyanophora paradoxa, a freshwater alga considered as a living fossil among photosynthetic organisms, represents an exciting model for studying the evolution of mitochondrial gene expression. As expected, fully mature tRNAs are released from primary transcripts to function in mitochondrial translation. We also show that these tRNAs take part in an mRNA processing punctuation mechanism in a non-conventional manner, leading to mRNA-tRNA hybrids with a CCA triplet at their 3'-extremities. In this case, tRNAs are probably used as stabilizing structures impeding the degradation of mRNA by exonucleases. From our data we propose that the present-day tRNA-like elements (t-elements) found at the 3'-terminals of mitochondrial mRNAs in land plants originate from true tRNAs like those observed in the mitochondria of this basal photosynthetic glaucophyte.