A horizontally transferred tRNACys gene in the sugar beet mitochondrial genome: evidence that the gene is present in diverse angiosperms and its transcript is aminoacylated

Of the two tRNACys(GCA) genes, trnC1-GCA and trnC2-GCA, previously identified in mitochondrial genome of sugar beet, the former is a native gene and probably a pseudo copy, whereas the latter, of origin unknown, is transcribed into a tRNA [tRNACys2(GCA)]. In this study, the trnC2-GCA sequence was mined from various public databases. To evaluate whether or not the trnC2-GCA sequence is located in the mitochondrial genome, the relative copy number of its sequence was assessed in a number of angiosperm species, using a quantitative real time PCR assay. The trnC2-GCA sequence was found to exist sporadically in the mitochondrial genomes of a wide range of angiosperms. The mitochondrial tRNACys2(GCA) species from sugar beet, spinach and cucumber were found to be aminoacylated, indicating that they may participate in translation. We also identified a sugar beet nuclear gene that encodes cysteinyl-tRNA synthetase, which is dual-targeted to mitochondria and plastids, and may aminoacylate tRNACys2(GCA). What is of particular interest is that trnC1-GCA and trnC2-GCA co-exist in the mitochondrial genomes of eight diverse angiosperms, including spinach, and that the spinach tRNACys1(GCA) is also aminoacylated. Taken together, our observations lead us to surmise that trnC2-GCA might have been horizontally transferred to a common ancestor of eudicots, followed by co-existence and dual-expression of trnC1-GCA and trnC2-GCA in mitochondria with occasional loss or inactivation of either trnC-GCA gene during evolution