Structure elucidation of new ascomycins produced by genetic engineering

Three new ascomycins produced by genetic engineering of Streptomyces hygroscopicus ATCC 14891 have been purified and characterized. Replacement of the 13-methoxyl group of ascomycin was accomplished by substitution of the corresponding acyltransferase domain of the polyketide synthase with a domain specific for either malonyl-CoA or methylmalonyl-CoA. The strain containing the methylmalonyl-specific acyltransferase domain produced a compound with properties consistent with those expected for 13-demethoxy-13-methylascomycin. NMR analysis revealed this material to be predominantly the cis amide rotamer, similar to ascomycin. The strain containing the malonyl-specific acyltransferase domain produced a mixture of two compounds, 13-demethoxyascomycin and the 9,14-hemiacetal isomer of 13-demethoxyascomycin, in nearly equal amounts. NMR analysis revealed both compounds to be predominantly the trans amide rotamers.