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The DNA sequence around the beginning of the genes coding for the large and small ribosomal RNAs in yeast mitochondria has been established. In order to determine the 5'-end points of the ribosomal RNAs, DNA fragments were labelled in vitro at a restriction site within each gene and hybridized with ribosomal RNA. The hybrids were then treated with S1 nuclease and the products analysed for size by gel electrophoresis. This enabled us to identify where in the determined DNA sequence the 21S ribosomal RNA and the precursor for 15S ribosomal RNA (15.5S rRNA) start, since both transcripts are initiated de novo (Levens et al. (1981) J.Biol.Chem., 256, 5226-5232). Comparison of the DNA sequences around the start points of transcription reveals the existence of a homologous stretch of 17 nucleotides. This conserved sequence may be an essential element of a promoter in mtDNA.