Bioconversion of milbemycin-related compounds: Isolation and utilization of non-producer, strain RNBC-5-51

A non-producing strain, the so-called strain RNBC-5-51 SANK 60198, was isol ated during a screening program of strain improvement in the milbemycin pro duction. Strain RNBC-5-51 indicated almost the same characteristics as thos e in the parent strain, that is, the abundant spore formation on agar media and the good growth in liquid media. But it does not produce any kind of m ilbemycins. In addition, strain RNBC-5-51 accumulated precursor-like compou nds of milbemycin-polyketide, the production of which were inhibited by the addition of cerulenin. In the bioconversion experiments, strain RNBC-5-51 converted milbemycin bet a(6) and A(4) to milbemycin alpha(14), and milbemycin beta(7) and A(3) to m ilbemycin alpha(11), respectively. This strain also converted milbemycin D and avermectin B-1a, to 26-(3-methyl-2-butenoyloxy)milbemycin D and 26-(3-m ethyl-2-butenoyloxy)avermectin B-1a, respectively. These results suggest that milbemycin alpha(11) is biosynthesized through t he same route as milbemycin alpha(14), and the mutated step in strain RNBC- 5-51 might be in the polyketide synthetic pathway of milbemycins. Strain RN BC-5-51 loses the ability for de novo synthesis of milbemycins, but it reta ins the ability to bioconvert the milbemycin skeleton. This strain might be useful for C-26 modification of milbemycin-related compounds.