The aminoacyl-tRNA synthetases (aaRS) are responsible for the accurate attachment of amino acids to their corresponding transfer RNA(s) (1). Each of the modern aaRSs (with a few exceptions) is selective for a single amino acid and tRNA (or tRNA isoacceptor set). These enzymes are some of the most ancient proteins fixed in modern organisms. The aaRSs are subdivided into two unrelated classes (class I and class II), based on class-defining active site structural features, and all aaRSs within a given class are believed to have evolved from a single common ancestor.
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