Improvement of milbemycin-producing Streptomyces bingchenggensis by rational screening of ultraviolet- and chemically induced mutants

Milbemycin antibiotics are produced by Streptomyces hygroscopicus subsp. aureolacrimosus and a newly isolated Streptomyces bingchenggensis, respectively. According to the biosynthetic pathway of milbemycins generated by S. hygroscopicus subsp. aureolacrimosus, a rational screening procedure with UV irradiation and N-methyl-N'-nitroso-N-nitrosoguanidine (NTG) mutation was performed to obtain high milbemycin-producing S. bingchenggensis. Aminoacetic acid (Glycine)-resistant mutants (AAr), propionate-resistant mutants (PRPr), streptomycin-resistant mutants (STRr) and 2-deoxy-d-glucose-resistant mutants (DOGr) were selected successively. A strain S. bingchenggensis BC-109-6 with AAr, PRPr, STRr and DOGr was obtained and its production of milbemycin A3 and A4 reached 1,450 μg/ml, which was 80% higher than that of the ancestral strain S. bingchenggensis BC-101-4. The subculture experiments indicated that the hereditary characteristic of high productivity of S. bingchenggensis BC-109-6 was stable. The production of milbemycin A3 and A4 by S. bingchenggensis BC-109-6 in a 50-l fermentor could reach 1,380 μg/ml after 360 h batch fermentation.