Transfer RNA genes in the mitochondrial genome from a liverwort, Marchantia polymorpha: the absence of chloroplast-like tRNAs.

Twenty-nine genes for 27 species of tRNAs were deduced from the complete nucleotide sequence of the mitochondrial genome from a liverwort, Marchantia polymorpha. One to three species of tRNA genes corresponded to each of 20 amino acids including three species for leucine and arginine, two species for serine and glycine, and one for the rest of the amino acids. Interestingly, all tRNA genes were located in the semicircle of the liverwort mitochondrial genome except for the trnY and trnR genes. The region containing these tRNA genes was originally duplicated, and two trnR genes have diverged from each other. On the other hand, trnY and trnfM are present as two identical copies. The G:U and U:N wobbling between the first nucleotide of the anticodon and the third nucleotide of the codon permit the 27 tRNA identified species to translate almost all codons. However, at least two additional tRNA genes, trnl-GAU for AUY codon and trnT-UGU for ACR codon, are required to read all codons used in the liverwort mitochondrial genome. All of the identified tRNA genes are 'native' in liverwort mitochondria, not 'chloroplast-like' tRNAs as are found in the mitochondria of higher plants. This result implies that the tRNA gene transfer from chloroplast to mitochondrial genome in higher plants has occurred after the divergence from bryophytes.