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Summary: The aminoacylation of tRNAs by aminoacyl-tRNA synthetases (ARSs) is a central reaction in biology. Multiple regulatory pathways use the aminoacylation status of cytosolic tRNAs to monitor and regulate metabolism. The existence of equivalent regulatory networks within the mitochondria is unknown. Here, we describe a functional network that couples protein synthesis to DNA replication in animal mitochondria. We show that a duplication of the gene coding for mitochondrial seryl-tRNA synthetase (SerRS2) generated in arthropods a paralog protein (SLIMP) that forms a heterodimeric complex with a SerRS2 monomer. This seryl-tRNA synthetase variant is essential for protein synthesis and mitochondrial respiration. In addition, SLIMP interacts with the substrate binding domain of the mitochondrial protease LON, thus stimulating proteolysis of the DNA-binding protein TFAM and preventing mitochondrial DNA (mtDNA) accumulation. Thus, mitochondrial translation is directly coupled to mtDNA levels by a network based upon a profound structural modification of an animal ARS. : Picchioni et al. report the architecture of a housekeeping enzyme that is essential for mitochondrial protein synthesis. This enzyme is a heterodimer that contains a catalytically active subunit and an inactive but essential monomer (SLIMP). SLIMP also interacts with a major mitochondrial protease to control mitochondrial DNA levels. Keywords: seryl-tRNA synthetase, LON, tRNA, mtDNA, mitochondria, translation, replication