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.