Phenylpyruvate decarboxylase (PPDC) ofAzospirillum brasilense, involved in the biosynthesis of the plant hormone indole-3-acetic acid and the anti-microbial compound phenylacetic acid, is a thiamine diphosphate-depend-ent enzyme that catalyses the nonoxidative decarboxylation of indole- and phenylpyruvate.