Engineering Streptomyces albus B4 for Enhanced Production of (R)-Mellein: A High-Titer Heterologous Biosynthesis Approach

The natural compound (R)-(−)-mellein exhibits antiseptic and fungicidal activities. We investigated its biosynthesis using the polyketide synthase encoded by SACE_5532 (pks8) from Saccharopolyspora erythraea heterologously expressed in Streptomyces albus B4, a chassis chosen for its fast growth, genetic manipulability, and ample large short-chain acyl-CoA precursor supply. High-level heterologous (R)-(−)-mellein yield was achieved by pks8 overexpression and duplication. The precursor supply pathways were strengthened by overexpression of SACE_0028 (encoding acetyl-CoA carboxylase) and four genes involved in β-oxidation (fadD, fadE, fadB, and fadA). Cell growth inhibition by (R)-(−)-mellein production at high concentration was relieved by in situ adsorption using Amberlite XAD16 resin. The final strain, B4mel12, produced (R)-(−)-mellein at 6395.2 mg/L in shake-flask fermentation. Overall, this is the first report of heterologous (R)-(−)-mellein synthesis in microorganism with a high titer. (R)-(−)-mellein prototype in this study opens a possibility for the overproduction of valuable melleins in S. albus B4.