Processing of yeast mitochondrial messenger RNAs at a conserved dodecamer sequence.

The yeast mitochondrial genes coding for cytochrome c oxidase subunit I ( COX1 ) and the ATPase subunits 8 and 6 are organized in one transcription unit. Precise mapping of RNA termini with S1 nuclease and primer extension analysis shows that the 3' end of the COX1 mRNA and the 5' end of the ATPase precursor RNA are juxtaposed within a conserved dodecamer sequence (5'- AAUAAUAUUCUU -3'). Sequence comparison reveals that this motif is present downstream of nearly all protein-encoding genes, including extragenic unassigned reading frames ( URFs ) and two URFs located within introns. Also the 3' terminus of an RNA species derived from the URF -containing intron of the large rRNA gene maps within such a dodecamer sequence. It is likely, therefore, that this motif serves as a processing point in the generation of mature mRNA. From a comparison of the various transcription units, we infer that RNAs that originate from an endonucleolytic cleavage at this sequence have stable 3' termini, while further processing of the 5' ends occurs. The efficiency of the initial cleavage varies between the different positions at which the motif is present.