Impact of thioesterase activity on tylosin biosynthesis in Streptomyces fradiae Original Research Article

Background: The polyketide lactone tylactone is produced in Streptomycesfradiae by the TyIG complex of five multifunctional proteins. As with other type I polyketide synthases, the enzyme catalysing the final elongation step (TyIGV) possesses an integral thioesterase domain that is believed to be responsible for chain termination and ring closure to form tylactone, which is then glycosylated to yield tylosin. In common with other macrolide producers, S. fradiae also possesses an additional thioesterase gene (orf5) located within the cluster of antibiotic biosynthetic genes. The function of the Orf5 protein is addressed here. Results: Disruption of orf5 reduced antibiotic accumulation in S. fradiae by atleast 85%. Under such circumstances, the strain accumulated desmycosin (demycarosyl-tylosin) due to a downstream polar effect on the expression of orf6, which encodes a mycarose biosynthetic enzyme. High levels of desmycosin production were restored in the disrupted strain by complementation with intact orf5, or with the corresponding thioesterase gene, nbmB, from S. narbonensis, but not with DNA encoding the integral thioesterase domain of TyIGV. Conclusions: Polyketide metabolism in S. fradiae is strongly dependent on thethioesterase activity encoded by orf5 (tylO). It is proposed that the TyIG complex might operate with a significant error frequency and be prone to blockage with aberrant polyketides. A putative editing activity associated with TyIO might be essential to unblock the polyketide synthase complex and thereby promote antibiotic accumulation.