The sandarazols are cryptic and structurally unique plasmid encoded toxins from a rare myxobacterium

Abstract

Soil dwelling bacteria such as myxobacteria defend themselves by using secondary metabolites to inhibit growth of competing microorganisms. In this work we describe a new plasmid found in Sandaracinus sp. MSr10575 named pSa001 spanning 209.7 kbp that harbors a cryptic secondary metabolite biosynthesis gene cluster (BGC). Activation of this BGC by homologous recombination mediated exchange of the native promoter sequence against a vanillate inducible system led to production and subsequent isolation and structure elucidation of novel secondary metabolites, the sandarazols A-G. The sandarazol structure contains intriguing features such as an -chlorinated ketone, an epoxyketone and a (2R)-2-amino-3- (N,N-dimethylamino)-propionic acid building block. In depth investigation of the underlying biosynthetic machinery led to a concise biosynthetic model for the new compound family, including several uncommon biosynthesis steps. The chlorinated congener sandarazol C shows an IC50 value of 0.5 {micro}M against HCT 116 cells and a MIC of 14 {micro}M against Mycobacterium smegmatis, which points at the sandarazols potential function as defensive secondary metabolites or toxins. The sandara-zols BGC location on pSa001 is one of the very few example of large multimodular BGCs on a replicative plasmid, whose existence points at the mechanism of horizontal gene transfer events of entire multimodular BGCs to exchange chemical warfare capabilities between bacterial species.