Innovative approach for improvement of an antibiotic- overproducing industrial strain of Streptomyces albus

Working with a Streptomyces albus strain that had previously been bred to produce industrial amounts (10 mg/ml) of salinomycin, we demonstrated the efficacy of introducing drug resistance-producing mutations for further strain improvement. Mutants with enhanced salinomycin production were detected at a high incidence (7 to 12%) among spontaneous isolates resistant to streptomycin (Str(r)), gentamicin, or rifampin (Rif(r)). Finally, we successfully demonstrated improvement of the salinomycin productivity of the industrial strain by 2.3-fold by introducing a triple mutation. The Strr mutant was shown to have a point mutation within the rpsL gene (encoding ribosomal protein S12). Likewise, the Rif(r) mutant possessed a mutation in the rpoB gene (encoding the RNA polymerase 0 subunit). Increased productivity of salinomycin in the Strr mutant (containing the K88R mutation in the S12 protein) may be a result of an aberrant protein synthesis mechanism. This aberration may manifest itself as enhanced translation activity in stationary-phase cells, as we have observed with the poly (U)-directed cell-free translation system. The K88R mutant ribosome was characterized by increased 70S complex stability in low Mg2+ concentrations. We conclude that this aberrant protein synthesis ability in the Strr mutant, which is a result of increased stability of the 70S complex, is responsible for the remarkable salinomycin production enhancement obtained.