Structure of a precursor to the yeast mitochondrial tRNAMetf. Implications for the function of the tRNA synthesis locus.

A transcript from the yeast mitochondrial tRNAMetf gene has been isolated from a petite deletion mutant, ND40. RNA sequence analysis demonstrates that it has a 5' unprocessed extension of 28 nucleotides, and capping experiments with guanylyltransferase reveal that the first nucleotide has a 5' di- or triphosphate. A comparison of the RNA sequence with the tRNAMetf gene sequence reported here shows that transcription initiation occurs at a sequence homologous to a nonanucleotide segment implicated as a promoter element of yeast mitochondrial DNA. The 3' end of this transcript is identical to the mature tRNAMetf and carries a CCA sequence. The transcript can be processed in vitro to yield a mature tRNAMetf and thus appears to be a bona fide tRNA precursor. This tRNA precursor accumulates in the absence of the mitochondrial tRNA synthesis locus whereas mature tRNAMetf can be made from the same gene in the presence of the locus. This data provides clear and convincing evidence that the synthesis locus codes for a 5' tRNA processing function.