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Two Types of Binding of Erythromycin to Ribosomes from Antibiotic-sensitive and -resistant Bacillus subtilis 168

From the ¹ From the Department of Biochemistry, Case Western Reserve University School of Medicine, Cleveland, Ohio 44106

The reversible binding of erythromycin to ribosomes of Bacillus subtilis 168 has been studied with two bacterial strains, one sensitive to low levels of the antibiotic and the other a relatively resistant mutant. Each ribosome from the sensitive strain binds approximately 1 molecule of erythromycin when a concentration of 4 x 10^-7 m is reached. As the concentration of antibiotic is raised to 10^-5 m no further binding takes place. At still higher concentrations, multiple interactions of erythromycin and the ribosomes occur. In contrast to the 1st molecule of erythromycin, which is specifically bound to the 50 S ribosomal subunit, the additional antibiotic molecules are bound nonspecifically to either the 30 S or 50 S particles. When resistant ribosomes are studied, a single molecule of erythromycin can bind to each 50 S subunit, so long as the concentration of antibiotic is kept below 10^-5 m. At higher concentrations, multiple interactions can take place, as with sensitive ribosomes. The main difference between sensitive and resistant ribosomes is that, with the former, the 1st erythromycin molecule is bound with sufficiently high affinity to produce a discontinuity in the curve relating the amount bound and the concentration. The 1st molecule of antibiotic is bound to each type of ribosome in the concentration range believed to exist within the cell when multiplication (protein synthesis) is inhibited. With either sensitive or resistant ribosomes, it appears that it is the 1st bound molecule of antibiotic which affects bacterial protein synthesis. Measurements of erythromycin binding under equilibrium conditions have revealed association constants of 2.6 x 10⁷ and approximately 8 x 10⁵ m^-1 for sensitive and resistant ribosomes, respectively. Thus the mutant B. subtilis is probably resistant to erythromycin because of some change in ribosomal structure resulting in a lowered binding affinity for this antibiotic.

Binding of the 1st molecule of erythromycin to sensitive ribosomes is dependent on the presence of monovalent cations (40 mm NH₄⁺ or K⁺). The bound erythromycin is freely exchangeable with erythromycin of the medium, and the binding is unaffected by the presence of tetracycline, chlortetracycline, or chloramphenicol. Implications regarding the identity of the site (donor or receptor) occupied by erythromycin are discussed.

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