The CBP2 gene from Saccharomyces douglasii is a functional homologue of the Saccharomyces cerevisiae gene and is essential for respiratory growth in the presence of a wild-type (intron-containing) mitochondrial genome

Interconnected roles of fungal nuclear- and intron-encoded maturases: at the crossroads of mitochondrial intron splicing

Group I and II introns are large catalytic RNAs (ribozymes) that are frequently encountered in fungal mitochondrial genomes. The discovery of respiratory mutants linked to intron splicing defects demonstrated that for the efficient removal of organellar introns there appears to be a requirement of protein splicing factors. These splicing factors can be intron-encoded proteins with maturase activities that usually promote the splicing of the introns that encode them (cis-acting) and/or nuclear-encoded factors that can promote the splicing of a range of different introns (trans-acting). Compared to plants organellar introns, fungal mitochondrial intron splicing is still poorly explored, especially in terms of the synergy of nuclear factors with intron-encoded maturases that has direct impact on splicing through their association with intron RNA. In addition, nuclear-encoded accessory factors might drive the splicing impetus through translational activation, mitoribosome assembly, and phosphorylation-mediated RNA turnover. This review explores protein-assisted splicing of introns by nuclear and mitochondrial-encoded maturases as a means of mitonuclear interplay that could respond to environmental and developmental factors promoting phenotypic adaptation and potentially speciation. It also highlights key evolutionary events that have led to changes in structure and ATP-dependence to accommodate the dual functionality of nuclear and organellar splicing factors.

The Cbp2 protein stimulates the splicing of the omega intron of yeast mitochondria

The Cbp2 protein is encoded in the nucleus and is required for the splicing of the terminal intron of the mitochondrial COB gene in Saccharomyces cerevisiae . Using a yeast strain that lacks this intron but contains a related group I intron in the precursor of the large ribosomal RNA, we have determined that Cbp2 protein is also required for the normal accumulation of 21S ribosomal RNA in vivo . Such strains bearing a deletion of the CBP2 gene adapt slowly to growth in glycerol/ethanol media 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 intron in mitochondria nor is it required in vitro at magnesium concentrations >5 mM. A similar intron exists in the large ribosomal RNA (LSU) gene of Saccharomyces douglasii . This intron does need Cbp2 for catalytic activity in physiologic magnesium. Similarities between the LSU introns and COB intron 5 suggest that Cbp2 may recognize conserved elements of the these two introns, and protein-induced UV crosslinks occur in similar sites in the substrate and catalytic domains of the RNA precursors.