Heterologous production of chlortetracycline in an industrial grade Streptomyces rimosus host.

High-yielding industrial Streptomyces producer is usually obtained by multiple rounds of random mutagenesis and screening. These strains have great potential to be developed as the versatile chassis for the discovery and titer improvement of desired heterologous products. Here, the industrial strain Streptomyces rimosus 461, which is a high producer of oxytetracycline, has been engineered as a robust host for heterologous expression of chlortetracycline (CTC) biosynthetic gene cluster. First, the industrial chassis strain SR0 was constructed by deleting the whole oxytetracycline gene cluster of S. rimosus 461. Then, the biosynthetic gene cluster ctc of Streptomyces aureofaciens ATCC 10762 was integrated into the chromosome of SR0. With an additional constitutively expressed cluster-situated activator gene ctcB, the CTC titer of the engineering strain SRC1 immediately reached 1.51 g/L in shaking flask. Then, the CTC titers were upgraded to 2.15 and 3.27 g/L, respectively, in the engineering strains SRC2 and SRC3 with the enhanced ctcB expression. Further, two cluster-situated resistance genes were co-overexpressed with ctcB. The resultant strain produced CTC up to 3.80 g/L in shaking flask fermentation, which represents 38 times increase in comparison with that of the original producer. Overall, SR0 presented in this study have great potential to be used for heterologous production of tetracyclines and other type II polyketides.