Strain improvement of a mildiomycin producer,Streptoverticillium rimofaciens B-98891

Summary Aminopterin (10 g/ml) was found to inhibit the formation of 5-hydroxymethylcytosine (HMC), a constituent of mildiomycin, without affecting the growth ofStreptoverticillium rimofaciens. This was available for selecting high-producing mutants.d-Cycloserine caused its morphological mutations at high frequency. In addition, mildiomycin (MIL) production varied widely among the strains picked up from colonies that developed on agar medium containing cycloserine at the inhibitory concentration to the growth. Consequently, we selected the mutants which were capable of producing MIL on agar medium containing 10 g/ml of aminopterin, among mutants enriched by cycloserine. A high-producing mutant thus obtained, C_R⁴-257, exhibited higher enzymatic activity of the HMC formation and higher resistance todl-serine hydroxamate than the original strain.l-Canavanine resistant mutants were furthermore selected to enhance the biosynthetic activity of the arginine-like moiety of MIL. Among them, we finally obtained an excellent mutant, CV^R-48, with an MIL production 2.6 times that of the original strain,S. rimofaciens B-98891.