THE YEAST NUCLEAR GENE DSS1 WHICH CODES FOR A PUTATIVE RNASE II, IS NECESSARY FOR THE FUNCTION OF THE MITOCHONDRIAL DEGRADOSOME IN PROCESSING AND TURNOVER OF RNA

The yeast nuclear gene DSS1 codes for a mitochondrial protein containi ng regions of homology to bacterial RNase II and can act as a multicop y suppressor of a deletion of the SUV3 gene, which encodes an RNA heli case. In order to establish the function of the DSS1 gene in mitochond rial biogenesis we studied RNA metabolism in yeast strains disrupted f or SUV3 or DSS1. The results indicate that in the absence of DSS1 the in vitro activity of 3'-5' exoribonuclease is abolished and mitochondr ial translation is blocked. In disruption strains harboring intronless mitochondrial genomes steady-state levels of COB mRNA and 16S rRNA we re very low, while in the presence of a mitochondrial genome containin g the omega intron in the 21S rRNA gene the excised intron accumulates . Moreover we observed an accumulation of precursors of 21S rRNA and t he VAR1 mRNA. All these phenotypes are virtually identical to those of strains in which SUV3 is disrupted. We suggest that the DSS1 gene pro duct, like the SUV3 gene product, is a subunit of the yeast mitochondr ial degradosome (mtEXO), and that this protein complex participates in intron-independent turnover and processing of mitochondrial transcrip ts. In addition our studies exclude any role for the NUC1 nuclease in the these phenomena.