4-Formylaminooxyvinylglycine (FVG) is an oxyvinylglycine-type nonproteinogenic amino acid that inhibits seed germination and the fire blight pathogen Erwinia amylovora. Despite the simple chemical structure of FVG, its biosynthesis has remained obscure for nearly a decade. Herein, we characterize the amidinotransferase GvgD and pyridoxal 5′-phosphate (PLP) dependent enzyme GvgH in vitro. GvgD catalyzes a reversible amidinotransfer reaction between l-canaline and l-canavanine. GvgH is supposed to install the aminooxy group in FVG via an elimination–addition reaction. The γ-elimination of O-succinylhomoserine and O-acetylhomoserine is validated to yield 2-aminobut-3-enoic acid. Besides, 2-mercaptoethanol acts as a non-specific substrate of GvgH for the subsequent addition, although the true substrate remains elusive. This work provides important insights into the biosynthesis of FVG.