The yeast nuclear gene DSS1 codes for a mitochondrial protein containing regions of homology to bacterial RNase II and can act as a multicopy suppressor of a deletion of the SUV3 gene, which encodes an RNA helicase. In order to establish the function of the DSS1 gene in mitochondrial biogenesis we studied RNA metabolism in yeast strains disrupted for SUV3 or DSS1. The results indicate that in the absence of DSS1 the in vitro activity of 3'-5' exoribonuclease is abolished and mitochondrial translation is blocked. In disruption strains harboring intronless mitochondrial genomes steady-state levels of COB mRNA and 16S rRNA were very low, while in the presence of a mitochondrial genome containing the omega intron in the 21S rRNA gene the excised intron accumulates. Moreover we observed an accumulation of precursors of 21S rRNA and the VAR1 mRNA. All these phenotypes are virtually identical to those of strains in which SUV3 is disrupted. We suggest that the DSS1 gene product, like the SUV3 gene product, is a subunit of the yeast mitochondrial degradosome (mtEXO), and that this protein complex participates in intron-independent turnover and processing of mitochondrial transcripts. In addition our studies exclude any role for the NUC1 nuclease in these phenomena.
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