Native acridone synthases I and II from Ruta graveolens L. form homodimers

Acridone synthase II cDNA was cloned from irradiated cell suspension cultures of Ruta graveolens L. and expressed in Escherichia coli. The translated polypeptide of Mr 42,681 revealed a high degree of similarity to heterologous chalcone and stilbene synthases (70-75%), and the sequence was 94% identical to that of acridone synthase I cloned previously from elicited Ruta cells. Highly active recombinant acridone synthases I and II were purified to apparent homogeneity by a four-step purification protocol, and the affinities to N-methylanthraniloyl-CoA and malonyl-CoA were determined. The molecular mass of acridone synthase II was estimated from size exclusion chromatography on a Fractogel EMD BioSEC (S) column at about 45 kDa, as compared to a mass of 44 +/- 3 kDa found for the acridone synthase I on Superdex 75. Nevertheless, the sedimentation analysis by ultracentrifugation revealed molecular masses of 81 +/- 4 kDa for both acridone synthases. It is proposed, therefore, that the acridone synthases of Ruta graveolens are typical homodimeric plant polyketide synthases.