Non-canonical Biosynthesis of the Brexane-Type Bishomosesquiterpene Chlororaphen through Two Consecutive Methylation Steps in Pseudomonas chlororaphis O6 and Variovorax boronicumulans PHE5-4

Abstract

A non-canonical biosynthetic pathway furnishing the first natural brexane-type bishomosesquiterpene (chlororaphen, C₁₇ H₂₈ ) was elucidated in the γ-proteobacterium Pseudomonas chlororaphis O6. A combination of genome mining, pathway cloning, in vitro enzyme assays, and NMR spectroscopy revealed a three-step pathway initiated by C10 methylation of farnesyl pyrophosphate (FPP, C₁₅ ) along with cyclization and ring contraction to furnish monocyclic γ-presodorifen pyrophosphate (γ-PSPP, C₁₆ ). Subsequent C-methylation of γ-PSPP by a second C-methyltransferase furnishes the monocyclic α-prechlororaphen pyrophosphate (α-PCPP, C₁₇ ), serving as the substrate for the terpene synthase. The same biosynthetic pathway was characterized in the β-proteobacterium Variovorax boronicumulans PHE5-4, demonstrating that non-canonical homosesquiterpene biosynthesis is more widespread in the bacterial domain than previously anticipated.

Keywords: Brexanes; Homoterpenes; Natural Products; Non-Canonical Pathway; Structure Elucidation.