Yeast DEAD Box Protein Mss116p Is a Transcription Elongation Factor That Modulates the Activity of Mitochondrial RNA Polymerase

DEAD box proteins have been widely implicated in regulation of gene expression. Here, we show that the yeastSaccharomyces cerevisiae DEAD box protein Mss116p, previously known as a mitochondrial splicing factor, also acts as a transcription factor that modulates the activity of the single-subunit mitochondrial RNA polymerase encoded byRPO41 . Binding of Mss116p stabilizes paused mitochondrial RNA polymerase elongation complexesin vitro and favors the posttranslocated state of the enzyme, resulting in a lower concentration of nucleotide substrate required to escape the pause; this mechanism of action is similar to that of elongation factors that enhance the processivity of multisubunit RNA polymerases. In a yeast strain in which the RNA splicing-related functions of Mss116p are dispensable, overexpression ofRPO41 orMSS116 increases cell survival from colonies that were exposed to low temperature, suggesting a role for Mss116p in enhancing the efficiency of mitochondrial transcription under stress conditions.