Molecular cloning, functional expression and characterization of d-limonene synthase from Schizonepeta tenuifolia

Limonene is one of the most simple cyclic monoterpenes, and two enantiomers, d- and l-limonene occur due to the chiral carbon at 4-position. Cyclization of GPP into limonene is catalyzed by the limonene synthase, and some l-limonene synthase cDNAs have already been cloned from several species of plants, mainly from Labiatae family. However, the d-limonene synthase gene has not yet been obtained, therefore, no information is available on the molecular mechanism of stereochemical regulation in limonene formation. To resolve this, we cloned the d-limonene synthase gene (dLMS) from Schizonepeta tenuifolia (Labiatae) by a reverse genetic approach, and we found that both d- and l-limonene synthase share similar features such as a transit peptide, an arginine rich domain, and a metal cation binding site in their structures. Here, we report on the cloning of dLMS, and the putative stereochemical regulation mechanism is discussed based on the comparison of the deduced amino acid sequence of dLMS with those of known l-limonene synthases.