Acyltransferase Domain Exchange between Two Independent Type I Polyketide Synthases in the Same Producer Strain of Macrolide Antibiotics

Streptomyces graminofaciens A-8890 produces two macrolide antibiotics, FD-891 and virustomycin A, both of which show significant biological activities. In this study, we identified the virustomycin A biosynthetic gene cluster, which encodes type I polyketide synthases (PKSs), ethylmalonyl-CoA biosynthetic enzymes, methoxymalony-acyl carrier protein biosynthetic enzymes, and post-PKS modification enzymes. Next, we demonstrated that the acyltransferase domain can be exchanged between the Vsm PKSs and the PKSs involved in FD-891 biosynthesis (Gfs PKSs), without encountering supply problems of the unique extender units. We exchanged the malonyltransferase domain in the loading module of Gfs PKS with the ethylmalonyltransferase domain and the methoxymalonyltransferase domain of Vsm PKSs. Consequently, the expected two-carbon elongated analog 26-ethyl-FD-891 was successfully produced with a titer comparable to FD-891 production by the wild type; however, exchange with the methoxymalonyltransferase domain did not produce any FD-891 analogs. Furthermore, 26-ethyl-FD-891 showed potent cytotoxic activity against HeLa cells, like natural FD-891.