All tRNAHis 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 tRNAHis 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, tRNAHis with or without a G–1 are found in a plant mitochondrial tRNA fraction. Furthermore, a recombinant Arabidopsis mitochondrial RNase P can cleave tRNAHis 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 tRNAHis 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 tRNAHis with a G–1 are recovered. This shows that a previously unreported tRNAHis guanylyltransferase activity is present in plant mitochondria.
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