During the biosynthesis of SF2575, a tetracycline antibiotic, the ketone of C-4 in 4-keto-anhydrotetracycline (4-keto-ATC, 2) is reduced by SsfF to form (R)-4-hydroxyl-ATC (7) which can be further transformed to the final product. According to the previous study, gene cluster tjh mediates the production of a series of new tetracyclines. Three new compounds in ΔtjhO5::2R mutant which are structurally similar with 4-keto-ATC but with (S)-C-4 were obtained. TjhD2 with high similarity with SsfF revealed by bioinformatic analysis indicates that these two enzymes may exhibit the similar enzymatic function. Two new tetracyclines via complementation of ssfF encoding C-4 ketone reductase into ΔtjhD2 mutant were successfully obtained. C-4 of these two compounds is all isomerized and 15 is the aglycone of 16. The combinatorial biosynthesis strategy not only successfully discovers two new natural products, but also verifies the function of TjhD2. Moreover, although TjhD2 and SsfF possess the similar catalytic function, they mediate the production of compounds with different configuration. These results may be applied in modification of tetracyclines and lay the foundation for further exploring the specific mechanism of stereoselective reduction of ketoreductases.

Key words: combinatorial biosynthesis, biosynthesis, type II polyketides, tetracycline, isomerization