A fused hybrid enzyme of 8-hydroxygeraniol oxidoreductase (8HGO) from Gardenia jasminoides and iridoid synthase (ISY) from Catharanthus roseus significantly enhances nepetalactol and iridoid production

The operation of 8HGO–ISY fusion enzymes can increase nepetalactol flux to iridoid biosynthesis, and the Gj8HGO–CrISY expression in Gardenia jasminoides indicates that seco-iridoids and closed-ring iridoids share a nepetalactol pool.

Nepetalactol is a common precursor of (seco)iridoids and their derivatives, which are a group of noncanonical monoterpenes. Functional characterization of an 8HGO (8-hydroxygeraniol oxidoreductase) from Catharanthus roseus, a seco-iridoids producing plant, has been reported; however, the 8HGO from G. jasminoides with plenty of closed-ring iridoids remains uninvestigated. In this work, a Gj8HGO was cloned and biochemically characterized. In addition, the relatively low production of nepetalactol in plants and engineered microbial host is likely to be attributed to the fact that Cr8HGO and CrISY (iridoid synthase) are substrate-promiscuous enzymes catalyzing unexpected substrates to the undesired products. Herein, a bifunctional enzyme consisting of an 8HGO fused to an ISY was designed for the proximity to the substrate and recycling of NADP⁺ and NADPH cofactor to reduce the undesired intermediate in the synthesis of nepetalactol. Of four fusion enzymes (i.e., Gj8HGO–GjISY, Gj8HGO–GjISY2, Gj8HGO–GjISY4, and Gj8HGO–CrISY), interestingly, only the last one can enable cascade reaction to form cis-trans-nepetalactol. Furthermore, we establish a reliable Agrobacterium-mediated transformation system. The expression of Gj8HGO–CrISY in G. jasminoides led to a significant enhancement of nepetalactol production, about 19-fold higher than that in wild-type plants, which further resulted in the twofold to fivefold increase of total iridoids and representative iridoid such as geniposide, indicating that seco-iridoids in C. roseus and closed-ring iridoids in G. jasminoides share a nepetalactol pool. All results suggest that 8HGO and ISY can be manipulated to maximize metabolic flux for nepetalactol and iridoid production.