Increasing Ascomycin Yield in Streptomyces Hygroscopicus var. Ascomyceticus by Using Polyhydroxybutyrate as an Intracellular Carbon Supply Station
Background: Ascomycin is a multifunctional antibiotic produced by Streptomyces hygroscopicus var. ascomyceticus. As a secondary metabolite, the production of ascomycin is often limited by the shortage of precursors during the late fermentation phase. Polyhydroxybutyrate is an intracellular polymer accumulated by various microorganisms. The development of polyhydroxybutyrate as a useful carbon reservoir for precursor synthesis is of great significance for improving the yield of
... f ascomycin.Results: The fermentation characteristics of the parent strain S. hygroscopicus var. ascomyceticus FS35 showed that the accumulation and decomposition of polyhydroxybutyrate were respectively correlated with the growth of strain and the production of ascomycin. The co-overexpression of exogenous polyhydroxybutyrate synthesis gene phaC and native polyhydroxybutyrate decomposition gene fkbU increased both strain biomass and ascomycin yield. Comparative transcription analysis showed that the storage of polyhydroxybutyrate during the exponential phase accelerated biosynthesis processes by stimulating the utilization of carbon sources, and the decomposition of polyhydroxybutyrate during the stationary phase increased the biosynthesis of ascomycin precursors by enhancing metabolic flux of primary pathways. The comparative analysis of cofactor concentration reflected that the biosynthesis of polyhydroxybutyrate depended on the supply of NADH pool. Under the condition of low sugar content in the later exponential phase, the optimization of carbon source addition further strengthened the polyhydroxybutyrate metabolism by increasing total concentration of cofactors. Finally, in the fermentation medium with 22 g/L starch and 52 g/L dextrin, the ascomycin yield of co-overexpression strain was increased to 626.30 mg/L, 2.11-fold higher than that of parent strain in the initial medium (296.29 mg/L). Conclusions: This paper reported for the first time that the polyhydroxybutyrate was beneficial to the growth of strain and the production of ascomycin by working as an intracellular carbon reservoir, storing as polymers when carbon sources are abundant and depolymerizing to monomers for the biosynthesis of precursors when carbon sources are insufficient. The successful application of polyhydroxybutyrate in increasing the output of ascomycin provided a new strategy for the high yield of other secondary metabolites.