THE CBP2 PROTEIN STIMULATES THE SPLICING OF THE OMEGA-INTRON OF YEASTMITOCHONDRIA

The Cbp2 protein is encoded in the nucleus and is required for the spl icing of the terminal intron of the mitochondrial COB gene in Saccharo myces cerevisiae. Using a yeast strain that lacks this intron but cont ains a related group I intron in the precursor of the large ribosomal RNA, we have determined that Cbp2 protein is also required for the nor mal accumulation of 21S ribosomal RNA in vivo. Such strains bearing a deletion of the CBP2 gene adapt slowly to growth in glycerol/ethanol m edia implying a defect in derepression. At physiologic concentrations of magnesium, Cbp2 stimulates the splicing of the ribosomal RNA intron in vitro. Nevertheless, Cbp2 is not essential for splicing of this in tron in mitochondria nor is it required in vitro at magnesium concentr ations >5 mM. A similar intron exists in the large ribosomal RNA (LSU) gene of Saccharomyces douglasii. This intron does need Cbp2 for catal ytic activity in physiologic magnesium. Similarities between the LSU i ntrons and COB intron 5 suggest that Cbp2 may recognize conserved elem ents of the these two introns, and protein-induced UV crosslinks occur in similar sites in the substrate and catalytic domains of the RNA pr ecursors.