Synthesis and processing of ribosomal RNA in isolated yeast mitochondria.

The synthesis and processing of the 15S and 21S rRNAs have been studied in isolated yeast mitochondria. When mitochondrial transcripts were labeled with [alpha-32p]UTP in an incubation mixture containing 50 microM ATP, the transcripts from the genes for the large and small ribosomal RNAs accumulated in the form of putative precursor molecules. The labeled pre-21S rRNA was converted to mature 21S rRNA during a chase period in the presence of 1 mM ATP. Thus, the maturation of 21S rRNA, a process which includes trimming at the 3' end and, in omega+ strains, the excision of a 1.1 kb intervening sequence, can occur in isolated mitochondria and appears to be dependent on ATP. In contrast, the maturation of 15S rRNA by the removal of approximately 80 nucleotides from the 5' end of a 15.5S transcript is severely restricted in isolated mitochondria, even in the presence of 2.5 mM ATP.