Cloning, expression and functional identification of a type III polyketide synthase gene from Huperzia serrata

A cDNA encoding novel type III polyketide synthase (PKS) was cloned and sequenced from young leaves of Chinese club moss Huperzia serrata (Thunb.) Trev. by RT-PCR using degenerated primers based on the conserved sequences of known CHSs, and named as H. serrata PKS2.  The terminal sequences of cDNA were obtained by the 3'- and 5'-RACE method.  The full-length cDNA of H. serrata PKS2 contained a 1 212 bp open reading frame encoding a 46.4 kDa protein with 404 amino acids.  The deduced amino acid sequence of H. serrata PKS2 showed 50%−66% identities to those of other chalcone synthase super family enzymes of plant origin.  The recombinant H. serrata PKS2 was functionally expressed in Escherichia coli with an additional hexahistidine tag at the N-terminus and showed unusually versatile catalytic potency to produce various aromatic tetraketides, including chalcones, benzophenones, phloroglucinols, and acridones.  In particular, the enzyme accepted bulky starter substrates N-methylanthraniloyl-CoA, and carried out three condensations with malonyl- CoA to produce 1, 3-dihydroxy-N-methylacridone.  Interestingly, H. serrata PKS2 lacks most of the consensus active site sequences with acridone synthase from Ruta graveolens (Rutaceae).