PROCESSING AND EDITING OF OVERLAPPING TRANSFER-RNAS IN HUMAN MITOCHONDRIA

Overlapping tRNA genes in mitochondria of many metazoans introduce a p roblem for the processing of such polycistronic primary transcripts. U sing runoff transcripts and an S100 extract from HeLa cell mitochondri a, the processing of the human mitochondrial tRNA(Tyr)/tRNA(Cys) precu rsor (carrying an overlap of one base) was investigated: tRNA(Cys) is released in its complete form carrying the overlapping residue at the first position, whereas tRNA(Tyr) lacks that nucleotide at the discrim inator position. Partial deletion of tRNA(Cys) Or complete replacement by a non-tRNA-like sequence does not alter the processing reaction an d indicates that the upstream tRNA(Tyr) alone is recognized by a 3'-en donuclease activity. The truncated 3'-end of this tRNA(Tyr) is then co mpleted in an editing reaction that incorporates the missing residue. The processing of this tRNA overlap seems to be species-specific, beca use an overlapping tRNA precursor (tRNA(Ser)(AGY)/tRNA(Leu)(CUN)) from opossum mitochondria is not recognized by the human extract. Because processing activities for overlapping and nonoverlapping tRNA precurso rs could not be separated, it seems that one general activity is respo nsible for the 3'-end processing of mitochondrial tRNAs and that this activity coevolved with the particular overlap between tRNA(Tyr) and t RNA(Cys) in human mitochondria, being unable to recognize overlaps bet ween other tRNAs.