Cloning and functional characterisation of a cis-muuroladiene synthase from black peppermint (Mentha x piperita) and direct evidence for a chemotype unable to synthesise farnesene

Using oligonucleotide primers designed to the known gene sequence of an (E)-beta-farnesene (E beta F) synthase, two cDNA sequences (MxpSS1 and MxpSS2) were cloned from a black peppermint (Mentha x piperita) plant. MxpSS1 encoded a protein with 96% overall amino acid sequence identity with the E beta F synthase. Recombinant MxpSS1 produced in Escherichia coli, after removal of an N-terminal thioredoxin fusion, had a K-m for FPP of 1.91 +/- 0.1 mu M and k(cat), of 0-18 s(-1) and converted farnesyl diphosphate (FPP) into four products, the major two being cis-muurola-3,5-diene (45%) and cis-muurola-4(14),5-diene (43%). This is the first cis-muuroladiene synthase, to be characterised. MxpSS2 encoded a protein with only two amino acids differing from E beta F synthase. Recombinant MxpSS2 protein showed no activity towards FPP. One of the two mutations, at position 531 (leucine in MxpSS2 and serine in E beta F synthase) was shown, by structural modelling to occur in the J-K loop, an element of the structure of sesquiterpene synthases known to be important in the reaction mechanism. Reintroduction of the serine at position 531 into MxpSS2 by site-directed mutagenesis restored E beta F synthase activity (K. for FPP 0.98 +/- 0.12 pM, k(cat) 0-1 s(-1)), demonstrating the crucial role of this residue in the enzyme activity. Analysis, by GC-MS, of the sesquiterpene profile of the plant used for the cloning, revealed that E beta F was not present, confirming that this particular mint chemotype had lost E beta F synthase activity due to the observed mutations. (c) 2005 Elsevier Ltd. All rights reserved.