Metabolic Network Analysis of Streptomyces tenebrarius , a Streptomyces Species with an Active Entner-Doudoroff Pathway

Abstract

Streptomyces tenebrarius is an industrially important microorganism, producing an antibiotic complex that mainly consists of the aminoglycosides apramycin, tobramycin carbamate, and kanamycin B carbamate. When S. tenebrarius is used for industrial tobramycin production, kanamycin B carbamate is an unwanted byproduct. The two compounds differ only by one hydroxyl group, which is present in kanamycin carbamate but is reduced during biosynthesis of tobramycin.13 C metabolic flux analysis was used for elucidating connections between the primary carbon metabolism and the composition of the antibiotic complex. Metabolic flux maps were constructed for the cells grown on minimal medium with glucose or with a glucose-glycerol mixture as the carbon source. The addition of glycerol, which is more reduced than glucose, led to a three-times-greater reduction of the kanamycin portion of the antibiotic complex. The labeling indicated an active Entner-Doudoroff (ED) pathway, which was previously considered to be nonfunctional in Streptomyces. The activity of the pentose phosphate (PP) pathway was low (10 to 20% of the glucose uptake rate). The fluxes through Embden-MeyerhofParnas (EMP) and ED pathways were almost evenly distributed during the exponential growth on glucose. During the transition from growth phase to production phase, a metabolic shift was observed, characterized by a decreased flux through the ED pathway and increased fluxes through the EMP and PP pathways. Higher specific NADH and NADPH production rates were calculated in the cultivation on glucose-glycerol, which was associated with a lower percentage of nonreduced antibiotic kanamycin B carbamate.The ability of actinomycetes to make secondary metabolites with different useful properties (antibacterials, antitumor agents, immunosuppressants, etc.) is widely exploited in the pharmaceutical industry. Two thirds of the antibiotics produced by microorganisms are made by actinomycetes. In particular, the Streptomyces genus is remarkable in this aspect, representing about 80% of the actinomycete antibiotics (16).Antibiotics are formed from specific precursors that are drained from the central carbon metabolism, and overproduction of antibiotics therefore requires that the precursors are supplied in sufficient quantities. Improvement of antibiotic production has traditionally been based on random mutagenesis methods, and in the future these methods will play an important role. However, metabolic engineering enables the introduction of rational changes to the central carbon metabolism to increase fluxes of precursors and cofactors to antibiotics. Metabolic flux analysis is a valuable tool in guiding metabolic engineering strategies, as it enables rapid phenotypic characterization of different mutants; through analysis of different mutants, one may gain insight into the correlation between antibiotic production and the fluxes through specific branches of the metabolic network.In this study, correlations between the primary and the secondary metabolism in the antibiotic...

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