Three candidate 2-(2-phenylethyl)chromone-producing type III polyketide synthases from Aquilaria sinensis (Lour.) Gilg have multifunctions synthesizing benzalacetones, quinolones and pyrones

Abstract

2-(2-Phenylethyl)chromones (PECs), with extensive pharmacological activities, have been found in only few plant species and are the characteristic components of agarwood which is an expensive fragrant and resinous wood. However, little is known about PEC biosynthesis pathway. Here, three type III polyketide synthases (PKSs), AsPKS3, AsPKS4, and AsPKS5, highly expressed in the agarwood layer were isolated from Aquilaria sinensis and characterized. The qRT–PCR results showed that the expression of them was strongly induced by salt, salicylic acid (SA), or methyl jasmonate (MeJA). In addition, their expression levels were consistent with PEC accumulation in calli under salt treatment. AsPKS proteins exhibited multiple activities in vitro, including catalyzing the synthesis of p-hydroxybenzalacetone, feruloylmethane, 4-hydroxy-N-methyl-2(1H)-quinolone, and 1-methyl-2-phenethylquinolin-4(1H)-one (which has a structure similar to those of PECs). Moreover, AsPKS4 or AsPKS5 could also catalyze the production of pyrones in vitro. The pH and temperature optima for the p-hydroxybenzalacetone or 4-hydroxy-N-methyl-2(1H)-quinolone production of AsPKSs were found to be different. Protein structure prediction showed that the three AsPKSs have large active site entrances allowing binding of bulky substrates such as N-methylanthraniloyl-CoA. Additionally, the three AsPKS proteins were localized in the cytoplasm and nucleus. This study not only facilitates the synthesis of structurally diverse benzalacetones, quinolones, and pyrones but also indicates prime enzymes involved in the biosynthesis of PECs for future studies.