THE DEXH BOX PROTEIN SUV3P IS A COMPONENT OF A YEAST MITOCHONDRIAL 3'-TO-5' EXORIBONUCLEASE THAT SUPPRESSES GROUP-I INTRON TOXICITY

The yeast mitochondrial protein Suv3p is a putative NTP-dependent RNA helicase. Here we report that in cells lacking Suv3p, there is an simi lar to 50-fold increase in the excised form of the group I intron omeg a of the mitochondrial 21S rRNA gene. Surprisingly, little mature 21S rRNA accumulates in those cells; instead, unligated 21S rRNA exons app ear. Intron overaccumulation could lead to spliced exon reopening via a reaction known to be catalyzed by group I introns in vitro. We also show that Suv3p is a functional component of a novel mitochondrial NTP -dependent 3'-to-5' exoribonuclease activity that can degrade group I intron RNAs. These findings account for group I intron overaccumulatio n in cells lacking Suv3p and define a novel function for putative RNA helicases in direct RNA degradation.