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

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Abstract
Acridone synthase II cDNA was cloned from irradiated cell suspension cultures of Ruta graveolens L. and expressed in Escherichia coli. The translated polypeptide of M r 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.