Repair of tRNAs in metazoan mitochondria

The integrity of 3′-ends of tRNAs is essential for aminoacylation and consequently for protein synthesis. The CCA-termini are generated and, if truncated by exonucleolytic activity, restored by tRNA nucleotidyltransferase. However, further truncations at the 3′-end can occur by exonuclease activity or during processing of overlapping tRNA primary transcripts in metazoan mitochondria. In the latter case, the upstream tRNA is released in a 3′-truncated form (lacking up to six bases) and subsequently completed. In human mitochondria, tRNATyr (missing the discriminator nucleotide A73) is completed by a discriminator adding activity followed by CCA addition. Since in vivo a high percentage of further 3′-terminally degraded human tRNATyr transcripts could be observed, it was tested in an in vitro system whether this repair mechanism for tRNA 3′-ends acts also on these further degraded tRNA versions. Additionally, 3′-truncated versions of two non-overlapping mitochon-drial tRNAs (tRNAThr and tRNAPhe) were examined. The results show that these transcripts can be repaired during incubation. A similar base incorporating activity was observed in mouse mitochondria, indi-cating that a repair mechanism for the 3′-end of several tRNAs exists in mitochondria of humans and possibly other metazoans which goes beyond the CCA addition