Enhancement and selective production of avermectin B by recombinants of Streptomyces avermitilis via intraspecific protoplast fusion

Among eight components of avermectin, B1 fractions have the most effective antiparasitic activities and the lowest level of toxic side-effects and are used widely in veterinary and agricultural fields. Intraspecific protoplast fusion between two strains of Streptomyces avermitilis, one an avermectin high producer (strain 76-05) and the other a genetically engineered strain containing the mutations aveD(-) and olmA(-) (strain 73-12) was performed for enhancement and selective production of avermectin B in the absence of oligomycin. Two recombinant strains (F23 and F29) were isolated and characterized with regards to the parental merits. F23 and F29 produced only the four avermectin B components with high yield and produced no oligomycin. The avermectin production of F23 and F29 was about 84.20% and 103.45% of the parental strain 76-05, respectively, and increased about 2.66-fold and 3.50-fold, respectively, compared to that of parental strain 73-12. F23 and F29 were genetically stable prototrophic recombinants and F29 was quite tolerant of fermentation conditions compared to avermectin high producer parental strain 76-05. The ability to produce avermectin B with high yield without the production of other avermectin components and oligomycin will make F23 and F29 useful strains for avermectin production. Strain F29's tolerance of fermentation conditions will also make it suitable for industrial applications.