Coexistence of nuclear DNA-encoded tRNAVal(AAC) and mitochondrial DNA-encoded tRNAVal(UAC) in mitochondria of a liverwort Marchantia polymorpha

The liverwort Marchantia polymorpha mitochondrial DNA encodes almost all tRNAs required for mitochondrial translation except for the isoleucine (AUU, AUC) and threonine (ACA, ACG) codons, while the missing tRNAs are supplied in part by the nucleus and imported in mitochondria, In this paper, we report a finding of two radically different nuclear tRNA(Val)(AAC) genes and import of the corresponding tRNA iso-acceptors in M.polymorpha mitochondria. This finding is surprising since the mtDNA encodes the gene for tRNA(Val)(UAC), which alone was considered sufficient for translating all four valine codons GUN by the U/N wobble mechanism. The present results suggest for the first time that the import of ncDNA-encoded tRNAs may result in decoding overlaps in plant mitochondria. The coexistence of nuclear DNA-encoded tRNA(Val)(AAC) and mitochondrial DNA-encoded tRNA(Val)(UAC) in liverwort mitochondria and the significance for the decoding mechanism as well as evolution of tRNA import are discussed.