Parallel loss of nuclear-encoded mitochondrial aminoacyl-tRNA synthetases and mtDNA-encoded tRNAs in Cnidaria.

Unlike most animal mitochondrial (mt) genomes, which encode a set of 22 transfer RNAs (tRNAs) sufficient for mt protein synthesis, those of cnidarians have only retained one or two tRNA genes. Whether the missing cnidarian mt-tRNA genes relocated outside the main mt chromosome or were lost remains unclear. It is also unknown what impact the loss of tRNA genes had on other components of the mt translational machinery. Here, we explored the nuclear genome of the cnidarian Nematostella vectensis for the presence of mt-tRNA genes and their corresponding mt aminoacyl-tRNA synthetases (mt-aaRS). We detected no candidates for mt-tRNA genes and only two mt-aaRS orthologs. At the same time, we found that all but one cytosolic aaRS appear to be targeted to mitochondria. These results indicate that the loss of mt-tRNAs in Cnidaria is genuine and occurred in parallel with the loss of nuclear-encoded mt-aaRS. Our phylogenetic analyses of individual aaRS revealed that although the nearly total loss of mt-aaRS is rare, aaRS gene deletion and replacement have occurred throughout the evolution of Metazoa.