Use of an inducible promoter for antibiotic production in a heterologous host

The biosynthetic gene cluster of the aminocoumarin antibiotic novobiocin comprises 20 coding sequences. Sixteen of them code for essential enzymes for novobiocin formation, transcribed in the form of a single 18-kb polycistronic mRNA. In the present study, we replaced the genuine promoter of this operon by the tetracycline-inducible promoter tcp830 and at the same time deleting the two pathway-specific positive regulator genes of novobiocin biosynthesis. The heterologous producer Streptomyces coelicolor M512 harboring the modified gene cluster produced, upon addition of 2 mg L-1 of the inducer compound anhydrotetracyline, 3.4-fold more novobiocin than strains carrying the unmodified cluster. A second tcp830 promoter was inserted in the middle of the 18-kb operon in order to ensure adequate transcription of the rearmost genes. However, this did not lead to a further increase of novobiocin formation, showing that a single tcp830 promoter was sufficient to achieve high transcription of all 16 genes of the operon. A high induction of novobiocin formation was achieved within a wide range of anhydrotetracyline concentrations (0.25–2.0 mg L-1). Growth of the strains was not affected by these concentrations. The inducer compound could be added either at the time of inoculation or at any other time up to mid-growth phase, always achieving a similar final antibiotic production. Therefore, the tcp830 promoter presents a robust, easy-to-use system for the inducible expression of biosynthetic gene clusters in heterologous hosts, independent from the genuine regulatory network.