New technology for the derivatization of thioamide ribosomally synthesized and post-translationally modified peptide (RiPP) compounds we previously developed was optimized for the derivatization of thiostrepton metabolites. These optimized procedures allowed for efficient thiostrepton derivatization. A heterologous expression system using an engineered Streptomyces avermitilis strain also exhibited high performance for metabolite production. We developed a streamlined procedure to generate new thiostrepton derivatives and apply their antimicrobial activities against gram-positive bacteria. Heterologous expression of 27 constructs with amino acid substitution mutation(s) in an engineered S. avermitilis strain produced 20 designed thiostrepton derivatives along with several previously undescribed analogs. Derivatives in which the second or fourth Ala residue in the thiostrepton skeleton was replaced with Ser or Val, respectively, exhibited improved antibacterial activity. The data suggest the present strategy is suitable for generating novel non-natural metabolites. Thiostrepton can induce expression of the tipA promoter of Streptomyces lividans 1326. It is highly intriguing that the antimicrobial activity of thiostrepton and its derivatives is correlated with induction of the tipA promoter.
