Biosynthesis and Heterologous Production of Vioprolides: Rational Biosynthetic Engineering and Unprecedented 4-Methylazetidinecarboxylic Acid Formation

Abstract

Vioprolides are a promising class of anticancer and antifungal lead compounds produced by the myxobacterium Cystobacter violaceus Cb vi35. Previously nothing had been reported about their biosynthesis, including the origin of the unusual 4-methylazetidinecarboxylic acid (MAZ) moiety. We describe the vioprolide biosynthetic gene cluster and solve the production obstacle by expression in three heterologous hosts. Starting from unstable production in the wild type at the single-digit mg L^-1 scale, we developed a stable host that eventually allowed for yields of up to half a gram per liter in fermenters. Gene inactivations coupled with isotope feeding studies identified an S-adenosylmethionine (SAM)-dependent enzyme and a methyltransferase as being responsible for the generation of the MAZ building block by a proposed mechanism unprecedented in bacteria. Furthermore, nonnatural vioprolide derivatives were generated via rational genetic engineering.

Keywords: azetidine; biosynthesis; heterologous expression; natural products; vioprolide.