Acquisition of Certain Streptomycin-Resistant (str) Mutations Enhances Antibiotic Production in Bacteria

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Abstract
Physiological differentiation (including antibiotic production) in microorganisms usually starts when cells encounter adverse environmental conditions and is frequently accompanied by an increase in the accumulation of intracellular ppGpp. We have found that the acquisition of certain streptomycin-resistant (str) mutations enables cells to overproduce antibiotics, demonstrating an increase in productivity 5- to 50-fold greater than that of wild-type strains. The frequency of such antibiotic-overproducing strains among the str mutants was shown to range from 3 to 46%, as examined with several strains of the genera Streptomyces,Bacillus, and Pseudomonas. Analysis of str mutants from Bacillus subtilisMarburg 168 revealed that a point mutation occurred within therpsL gene, which encodes the ribosomal protein S12, changing Lys-56 (corresponding to Lys-43 in Escherichia coli) to Asn, Arg, Thr, or Gln. Antibiotic productivity increased in a hierarchical manner depending upon which amino acid residue replaced Lys at this position. The strA1 mutation, a genetic marker frequently used for mapping, had no effect on antibiotic productivity even though it was found to result in an amino acid alteration of Lys-56 to Ile. Gene replacement experiments with thestr alleles demonstrated unambiguously that thestr mutation is responsible for the antibiotic overproductivity observed. These results offer a rational approach for improving the production of antibiotic (secondary metabolism) from microorganisms.

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