Acridone synthase has been purified from cell suspension cultures of Ruta graveolens using a combination of gel filtration and ion exchange chromatography. The purified enzyme has an apparent molecular weight of 69 kDa on gel filtration and a subunit structure on SDS-PAGE of 40 kDa. The apparent Km-values are 10.64 microM and 32.8 microM for N-methylanthraniloyl-CoA and malonyl-CoA, respectively. Tryptic digestion of the homogeneous acridone synthase was performed. Seven of the peptides were chosen for microsequencing. The homology of the amino acid sequences from this particular polypeptide and corresponding peptides from chalcone synthase 3 from garden pea amounted to 76%.