Site of Action of Mikamycins A and B in Polypeptide Synthesizing Systems

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Abstract
The mechanism of action of mikamycins A and B, inhibitors of protein synthesis, was investigated with E. coli ribosomal systems. In both poly U- and poly A- systems polypeptide synthesis by ribosomes charged with the respective polyribonucleotides was as sensitive to mikamycins A and B as that by ribosomes which had been preincu-bated with the antibiotics prior to the addition of polyribonucleotides. This fact suggests that both kinds of mikamycin may not affect the formation of the ribosome-polyribonucleotide complex. Mikamycin A inhibited the binding of several C14-aminoacyl-sRNA's tested to the ribosome-polyribonucleotide complex to varied extents. The binding of phenylalanyl- and lysyl-sRNA's was more markedly affected than that of prolyi-sRNA and the extent of such inhibitions of binding among these three systems was parallel to, though less than, that of the corresponding over-all polypeptide synthesis. On the contrary, mikamycin B did not significantly inhibit the attachment of any aminoacyl-sRNA tested to the active complex. The degree of inhibition by mikamycin A of polypeptide synthesis decreased when it was introduced to the reaction mixture later, whereas that by mikamycin B showed a tendency to increase with the delay of addition. These results suggest that mikamycin A primarily affects an early ribosomal phase of chain growth involving the binding of aminoacyl-sRNA and the initial peptide chain growth, whereas the site of action of mikamycin B is thought to be in somewhere after the phase attacked by mikamycin A.

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