MODE OF ACTION OF SIOMYCIN

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The incorporation of H³-lysine into Bacillus subtilis cells ceased almost immediately after the addition of siomycin, while the continued incorporation of C¹⁴-uridine and of C¹⁴-thymidine into the trichloroacetic acid-insoluble fraction of the cells was observed for at least a further 20 minutes. In bacterialcell free systems (Bacillus subtilis and Escherichia coli) siomycin inhibits polyadenylic acid directed polylysine synthesis and polyuridylic acid-directed polyphenylalanine synthesis. However, aminoacyl-transfer RNA synthesis was hardly affected. The "soluble siomycin", monothiomalic acid-siomycin A, was found to have an inhibitory action on bacterial protein biosynthesis, similar to and to roughly the same extent as siomycin A. In contrast to bacterial protein synthesis, C¹⁴-leucine incorporation into rabbit reticulocyte hemoglobin was not significantly decreased by the addition of siomycin. Protein biosynthesis in a cell-free system from rabbit reticuloeytes was also found to be highly resistant to this antibiotic. These results are in harmony with the previous observation that siomycin showed little toxicity to mammals.