Type II thioesterase improves heterologous biosynthesis of valinomycin in Escherichia coli

• We demonstrated the in vivo positive effect of a type II thioesterase (TEII) from a nonribosomal peptide synthetase. This is new, as so far this was only shown for TEII from polyketide synthases.
• The heterologous biosynthesis of valinomycin can be significantly increased by the coexpression of the cognate TEII in Escherichia coli.
• 13 mg L−1 of valinomycin was the highest yield currently reported in E. coli.

Heterologous expression of secondary metabolite biosynthesis pathways in a surrogate host, e.g. Escherichia coli, has emerged in recent years as an effective way to produce complex natural products. The nonribosomal peptide (NRP) antibiotic valinomycin has been recombinantly produced in E. coli through reconstitution of its biosynthetic pathway from the native producer Streptomyces tsusimaensis. In this study, a discrete protein type II thioesterase (TEII) encoded in the valinomycin gene cluster was coexpressed in the valinomycin producing E. coli strain. Valinomycin titers were significantly improved from 0.5 (without TEII coexpression) to 3.3 mg L−1, which demonstrates the reconstitutive function of TEII involved in NRP biosynthesis. Based on a flask scale fed-batch cultivation system, repeated feeding of the glucose polymer during the cultivation further increased cell density and valinomycin titer up to 55 (OD600) and 13 mg L−1, respectively. This indicates scalable high cell density cultivation in a bioreactor for overproduction of valinomycin will be a potential and feasible approach. In this work we present an in vivo example to show that TEII plays a positive role in heterologous valinomycin production.