Processing of precursors of 21S ribosomal RNA from yeast mitochondria.

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Abstract
The transcription and processing of mitochondrial 21S rRNA in a petite strain of S. cerevisiae was examined by EM analysis of R-loop hybrids and by hybridization of labeled mitochondrial DNA probes to RNA transferred to diazobenzyloxymethyl paper. The presence was shown of a large [5.1-5.4 kilobase (kb)] transcript that appears to be a precursor of mitochondrial 21S rRNA. This transcript contains sequences homologous to those of the mature 21S rRNA, to the intervening sequence presence in the gene, and to additional sequences at the 3'' end of the molecule. This precursor of 21S rRNA may be processed in 2 steps. The intron sequence is usually excised first, followed by removal of the extra 3'' sequences. In some cases, however, the 3'' extension is first removed and the intron sequence is then excised. Both pathways appear to lead to formation of the 3.1-kb mature 21S rRNA and a stable 1.2-kb intron transcript. Similar results were obtained with grande MH41-7B mitochondrial RNA by RNA transfer hybridization. A number of additional transcripts were observed that may be normal processing intermediates or may result from faulty cleavage-ligation during excision of the intervening sequence.