Splicing of Yeast aI5β Group I Intron Requires SUV3 to Recycle MRS1 via Mitochondrial Degradosome-promoted Decay of Excised Intron Ribonucleoprotein (RNP)

Yeast Suv3p is a member of the DEXH/D box family of RNA helicases and is a critical component of the mitochondrial degradosome, which also includes a 3' --> 5' exonuclease, Dss1p. Defects in the degradosome result in accumulation of aberrant transcripts, unprocessed transcripts, and excised group I introns. In addition, defects in SUV3 result in decreased splicing of the aI5 beta and bI3 group I introns. Whereas a role for Suv3p in RNA degradation is well established, the function of Suv3p in splicing of group I introns has remained elusive. It has been particularly challenging to determine if Suv3p effects group I intron splicing through RNA degradation as part of the degradosome, or has a direct role in splicing as a chaperone, because nearly all perturbations of SUV3 or DSS1 result in loss of the mitochondrial genome. Here we utilized the suv3-1 allele, which is defective in RNA metabolism and yet maintains a stable mitochondrial genome, to investigate the role of Suv3p in splicing of the aI5 beta group I intron. We provide genetic evidence that Mrs1p is a limiting cofactor for aI5 beta splicing, and this evidence also suggests that Suv3p activity is required to recycle the excised aI5 beta ribonucleoprotein. We also show that Suv3p acts indirectly as a component of the degradosome to promote aI5 beta splicing. We present a model whereby defects in Suv3p result in accumulation of stable, excised group I intron ribonucleoproteins, which result in sequestration of Mrs1p, and a concomitant reduction in splicing of aI5 beta.