Radiolabelling of the monate moiety in the study of pseudomonic acid biosynthesis

Abstract

Monic acid A was isolated from a Pseudomonas fluorescens fermentation in which pseudomonic acid A (PA) was the principal secondary metabolite. [3-14C]3-Hydroxy-3-methyl-glutaric acid (HMG) given early in the idiophase radiolabelled PA (1.1% incorporation), confirming experimentally the putative direct involvement of HMG in the biosynthesis of PA, but contributed relatively insignificant radiolabel to the monic acid extracted from the broth at the end of the fermentation. Ethionine inhibited (80%) PA biosynthesis and correspondingly reduced incorporation of [14C]HMG. In contrast, ethionine increased incorporation of [methyl-14C]methionine into PA and enhanced specific radioactivity of the antibiotic 8-fold. Ethionine inhibition of secondary metabolite methylations did not divert pseudomonate biosynthesis to give unusual analogues, implying that methylation of a putative pentaketide precursor of the monate moiety forms a vital intermediate of the pseudomonate pathway, but caused a new [14C]HMG-derived polar metabolite of biosynthetic interest to become evident.