Plant mitochondria use two pathways for the biogenesis of tRNAHis.

All tRNA(His) possess an essential extra G(-1) guanosine residue at their 5' end. In eukaryotes after standard processing by RNase P, G(-1) is added by a tRNA(His) guanylyl transferase. In prokaryotes, G(-1) is genome-encoded and retained during maturation. In plant mitochondria, although trnH genes possess a G(-1) we find here that both maturation pathways can be used. Indeed, tRNA(His) with or without a G(-1) are found in a plant mitochondrial tRNA fraction. Furthermore, a recombinant Arabidopsis mitochondrial RNase P can cleave tRNA(His) precursors at both positions G(+1) and G(-1). The G(-1) is essential for recognition by plant mitochondrial histidyl-tRNA synthetase. Whether, as shown in prokaryotes and eukaryotes, the presence of uncharged tRNA(His) without G(-1) has a function or not in plant mitochondrial gene regulation is an open question. We find that when a mutated version of a plant mitochondrial trnH gene containing no encoded extra G is introduced and expressed into isolated potato mitochondria, mature tRNA(His) with a G(-1) are recovered. This shows that a previously unreported tRNA(His) guanylyltransferase activity is present in plant mitochondria.

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