tRNA wybutosine-synthesizing protein 3, known also as "tRNAPhe 7-[(3-amino-3-carboxypropyl)-4-demethylwyosine37-"N"4]-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 α-leucine esters. The enzyme catalyzes the following reaction,
4-demethyl-7-[(3S)-3-amino-3-carboxypropyl]wyosine(37) + S-adenosyl-L-methionine = 7-[(3S)-3-amino-3-carboxypropyl]wyosine(37) + S-adenosyl-L-homocysteine + H+
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.