BACKGROUND Many natural anthracycline glycoside antibiotics (AGAs), e.g. daunorubicin, are widely used as anticancer drugs in clinical practice, and their biosynthesis have been extensively studied. Epirubicin is a semi-synthesized derivative of daunorubicin, and more widely used in clinics due to its higher effectiveness and weaker toxicity than natural AGAs. However, the overall conversion rate of epirubicin from daunorubicin is only 15%. 4 '-Epidaunorubin is a crucial intermediate in semi-synthesis of epirubicin from daunorubicin. Much effort has been made to establish combinatorial biosynthesis approaches for production of 4 '-epidaunorubin, and 93 mg L-1 is the highest titer so far achieved. RESULTS CONCLUSION Daunorubicin-producing Streptomyces coeruleorubidus SL-7 were switched to 4 '-epidaunorubicin producer MSL201 by replacing dnmV with aveBIV. Then, continued to genetically modify MSL201 to increase production of 4 '-epidaunorubicin by in-frame knockout of dnrU and dnrX which catalyze conversion of daunorubicin to (13s)-13-dihydrodaunorubicin and baumycin-like compounds, respectively. The resulting strain MSL 203 was transformed by pSL206 containing a double aveBIV gene expression cassette to afford MSL 204, the 4 '-epidaunorubicin yield of which is 124.1 mg L-1, the highest yield of 4 '-epidaunorubicin ever obtained by fermentation methods. Knockout of the genes involved in consuming daunorubicin and increase of copy numbers of aveBIV gene can significantly improve the yield of 4 '-epidaunorubicin, giving the hitherto highest yield by a fermentation method. This result paves the way for commercial production of 4 '-epidaunorubicin via metabolic engineering approaches. (c) 2018 Society of Chemical Industry