Microsporidia are parasitic fungi-like organisms that invade the interior of living cells and cause chronic disorders in a broad range of animals, including humans. These pathogens have the tiniest known genomes among eukaryotic species, for which they serve as a model for exploring the phenomenon of genome reduction in obligate intracellular parasites. Here we report a case study to show an apparent effect of overall genome reduction on the primary structure and activity of aminoacyl-tRNA

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... etases, indispensable cellular proteins required for protein synthesis. We find that most microsporidian synthetases lack regulatory and eukaryote-specific appended domains and have a high degree of sequence variability in tRNA-binding and catalytic domains. In one synthetase, LeuRS, an apparent sequence degeneration annihilates the editing domain, a catalytic center responsible for the accurate selection of leucine for protein synthesis. Unlike accurate LeuRS synthetases from other eukaryotic species, microsporidian LeuRS is error-prone: apart from leucine, it occasionally uses its near-cognate substrates, such as norvaline, isoleucine, valine, and methionine. Mass spectrometry analysis of the microsporidium Vavraia culicis proteome reveals that nearly 6% of leucine residues are erroneously replaced by other amino acids. This remarkably high frequency of mistranslation is not limited to leucine codons and appears to be a general property of protein synthesis in microsporidian parasites. Taken together, our findings reveal that the microsporidian protein synthesis machinery is editing-deficient, and that the proteome of microsporidian parasites is more diverse than would be anticipated based on their genome sequences. Muller's ratchet | genome erosion | Microsporidia | aminoacyl-tRNA synthetases | mistranslation