Sls1 and Mtf2 mediate the assembly of the Mrh5C complex required for activation of cox1 mRNA translation.

Mitochondrial translation depends on mRNA-specific activators. In Schizosaccharomyces pombe, DEAD-box protein Mrh5, pentatricopeptide repeat (PPR) protein Ppr4, Mtf2, and Sls1 form a stable complex (designated Mrh5C) required for translation of mitochondrial DNA (mtDNA)-encoded cox1 mRNA, the largest subunit of the cytochrome c oxidase complex. However, how Mrh5C is formed and what role Mrh5C plays in cox1 mRNA translation have not been reported. To address these questions, we investigated the role of individual Mrh5C subunits in the assembly and function of Mrh5C. Our results revealed that Mtf2 and Sls1 form a subcomplex that serves as a scaffold to bring Mrh5 and Ppr4 together. Mrh5C binds to the small subunit of the mitoribosome (mtSSU), but each subunit could not bind to the mtSSU independently. Importantly, Mrh5C is required for the association of cox1 mRNA with the mtSSU. Finally, we investigated the importance of the signature DEAD-box in Mrh5. We found that the DEAD-box of Mrh5 is required for the association of Mrh5C and cox1 mRNA with the mtSSU. Unexpectedly, this motif is also required for the interaction of Mrh5 with other Mrh5C subunits. Altogether, our results suggest that Mrh5 and Ppr4 cooperate in activating the translation of cox1 mRNA. Our results also suggest that Mrh5C activates the translation of cox1 mRNA by promoting the recruitment of cox1 mRNA to the mtSSU.