A unique substrate specificity of PonAAS2, an aromatic aldehyde synthase, involved in a phytohormone auxin biosynthesis in a gall-inducing sawfly Euura sp. "Pontania".

The aromatic aldehyde synthase (AAS), PonAAS2, from the gall-inducing sawfly has been identified as a biosynthetic enzyme for indole-3-acetic acid (IAA), a key molecule of the plant hormone auxin, which is thought to play a role in gall induction. Unlike other insect AASs that convert Dopa, PonAAS2 uniquely converts L-tryptophan (Trp) into indole-3-acetaldehyde, a precursor of IAA. In this study, an examination of AAS enzymes from various insect species revealed that the ability to convert Trp has been acquired in only a very limited taxonomic group. Comparative analysis between PonAAS2 and DjAAS2 from a gall wasp showed that, despite having conserved substrate-recognition amino acids, they exhibit different substrate specificities. This difference likely arises from variations in how these enzymes' monomers interact during dimer formation, as demonstrated by amino acid substitution experiments and structural predictions.