Catalyzes the conversion of S-adenosyl-L-methionine (SAM) to carboxy-S-adenosyl-L-methionine (Cx-SAM). prephenate + S-adenosyl-L-methionine = 3-phenylpyruvate + carboxy-S-adenosyl-L-methionine + H2O Homodimer. Deletion mutant shows accumulation of 5-methoxyuridine (mo5U34) and 5-hydroxyuridine (ho5U34) in tRNA. Belongs to the class I-like SAM-binding methyltransferase superfamily. Cx-SAM synthase family. Was originally thought to be a transferase that mediates the conversion of mo5U(34) to cmo5U(34) in tRNAs.