tRNA wybutosine-synthesizing protein 3, known also as tRNA<sup>Phe</sup> 7-[(3-amino-3-carboxypropyl)-4-demethylwyosine<sup>37</sup>-N<sup>4</sup>]-methyltransferase, abbreviated to TYW3 is an S-adenosyl-L-methionine-dependent methyltransferase that is involved in the biosynthetic pathway of wybutosine, a hyper-modified guanosine possessing tricyclic base found at the 3'-position which is close to the anticodon of eukaryotic phenylalanine tRNA. TYW3 is believed to also methylate the carboxyl group of leucine to form ñ-leucine esters. The enzyme catalyzes the following reaction,
The modifications this enzyme makes are important for translational reading-frame maintenance. TYW3 is found in all eukaryotes and in some archaea, but not in bacteria.