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

Acridone synthase II cDNA was cloned from irradiated cell suspension cultur es of Ruta graveolens L. and expressed in Escherichia coli. The translated polypeptide of M-r 42681 revealed a high degree of similarity to heterologo us chalcone and stilbene synthases (70-75%), and the sequence was 94% ident ical to that of acridone synthase I cloned previously from elicited Ruta ce lls. Highly active recombinant acridone synthases I and II were purified to apparent homogeneity by a four-step purification protocol, and the affinit ies to N-methylanthraniloyl-CoA and malonyl-CoA were determined. The molecu lar mass of acridone synthase II was estimated from size exclusion chromato graphy 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. N evertheless, the sedimentation analysis by ultracentrifugation revealed mol ecular masses of 81+/-4 kDa for both acridone synthases. It is proposed, th erefore, that the acridone synthases of Ruta graveolens are typical homodim eric plant polyketide synthases. (C) 1999 Federation of European Biochemica l Societies.