The Schizosaccharomyces pombe DEAD-box protein Mss116 is required for mitoribosome assembly and mitochondrial translation.
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spMss116 is inside mitochondria, protected from proteinase K in intact organelles, and mostly carbonate-soluble with a small membrane-associated pool
"spMss116 was protected from proteinase K digestion in the absence of Triton X-100 (Fig. 1B), demonstrating that spMss116 is localized inside the mitochondria."
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In intron-containing mtDNA, deletion reduces spliced cox1 and cob1 mRNAs and increases intron-retaining precursors
"RT-PCR with cox1 and cob1 intron primers revealed that the levels of intronretaining cox1 and cob1 pre-mRNAs were increased in Δspmss116 cells"
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Other mt-mRNAs and the mt-rRNAs are not reduced in the deletion
"the levels of other mt-mRNAs and mt-rRNAs were unchanged or slightly increased in Δspmss116 cells"
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Splicing of the S. pombe mitochondrial introns depends on intron-encoded proteins made by mitochondrial translation, so a translation defect alone can reduce cox1 and cob1 mRNA
"relies on their intron-encoded proteins, whose synthesis was dependent on mitochondrial translation."
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The authors leave open whether spMss116 acts in splicing
"It remains to be determined whether spMss116 participates in splicing of mitochondrial introns."
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Synthesis of all mtDNA-encoded proteins is decreased (in vivo [35S] labelling with anisomycin), also in intronless mtDNA
"the levels of mtDNA-encoded proteins were substantially decreased in Δspmss116 [Δi] cells compared to WT[Δi] cells"
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The translation defect is global rather than mRNA-specific
"we demonstrate that the DEAD-box helicase spMss116 is required for global mitochondrial translation in S. pombe."
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spMss116 does not co-sediment with either mitoribosomal subunit or the monosome in sucrose gradients
"The results showed that spMss116-Myc did not cosediment with the mtSSU, mtLSU, or mitoribosome (Fig. 6A)."
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Deletion reduces assembled mitoribosomes and lowers both mtSSU (Mrp5, Rsm24) and mtLSU (Mrpl40) protein levels, also in intronless mtDNA
"Deletion of spmss116 led to a statistically significant reduction in levels of two mtSSU proteins Mrp5 and Rsm24, and two mtLSU proteins Mrpl40 and"
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The authors present the assembly role as a suggestion and propose mt-rRNA as a possible target
"Our study also suggests that spMss116 may participate in mitoribosome assembly."
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Budding-yeast Mss116 cannot substitute for spMss116
"overexpression of spmss116 but not scmss116 was able to restore growth of Δspmss116 cells on respiratory medium"
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The cox1-specific translational role of scMss116-Pet309 is taken by the Mrh5 complex in S. pombe
"Thus, it appears that in S. pombe, Mrh5C assumes the role of the scMss116-Pet309 complex in mediating cox1 mRNA translation in S. cerevisiae."
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S. pombe mtDNA encodes a mitoribosomal small-subunit protein, so a primary translation defect can itself impair mtSSU assembly
"a small mitochondrial ribosomal (mitoribosome) subunit (mtSSU) protein"