Although aminoacyl-tRNA synthetases catalyze the same chemical reaction, the individual enzymes have a wide range of sizes. Proteolytic digestion has yielded active catalytic fragments of two synthetases. A set of gene deletions in a large synthetase has been used successfully in the creation of a variety of enzyme fragments that have been studied individually; a fragment with about half of the total polypeptide is sufficient to aminoacylate tRNA in vivo. The results suggest that size polymorphism is caused by fusion, to a core catalytic segment, of variable amounts of additional polypeptide sequences. These sequences may serve to impart additional functions. For example, in one case, a synthetase binds to its own gene promoter and regulates transcription.