Enhancement of carrimycin production in Streptomyces spiramyceticus 54-IA by the reporter-guided selection method with traditional mutagenesis.

Carrimycin, a new drug approved in recent years, is produced by recombinant Streptomyces spiramyceticus harboring a heterogeneous 4″-O-isovaleryltransferase gene (ist). The regulatory gene bsm42 plays a crucial role in activation of carrimycin biosynthesis, its high expression can improve the carrimycin production. The xylE-neo reporter cassette under control of the the two different lengths of bsm42 promoters, F1R and F2R, can be used to monitor the expression of bsm42 to screen carrimycin high-yield mutants treated with the composite mutagenesis of plasma and ultraviolet radiation. We found that 47% of the mutants selected based on the higher expression of reporter genes driven by bsm42 promoters showed improved carrimycin production, and the shorter promoter F2R was more effective than promoter F1R. The high-yield mutant F2R-15 was about 13 times higher than that of 54-IA strain, and its titer reached 1010 ± 30 µg/mL. A comparative analysis of the transcriptome profiles between the F2R-15 mutant and 54-IA strains revealed 158 significantly differentially expressed genes. The upregulated genes were associated with macrolide precursor biosynthesis, macrolide inactivation, antibiotic transporters, and oxidative phosphorylation, while the most downregulated genes were related to primary biosynthetic metabolites and other secondary metabolites. These results demonstrate that the reporter-guided selection method combined with traditional mutagenesis is an effective strategy for obtaining stable carrimycin high-yield mutants.