摘要：

The reversible binding of erythromycin to ribosomes of Bacillus subtilis 168 has been studied with two bacterialstrains, 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 eryth- romycin when a concentration of 4 x 1OV M is reached.As the concentration of antibiotic is raised to lo-" M no further binding takes place. At still higher concentrations,multiple interactions of erythromycin and the ribosomesoccur. In contrast to the 1st molecule of erythromycin,which is specifically bound to the 50 S ribosomal subunit,the additional antibiotic molecules are bound nonspecificallyto either the 30 S or 50 S particles. When resistant ribo- somes are studied, a single molecule of erythromycin can bind to each 50 S subunit, so long as the concentration of anti- biotic is kept below 10-j M. At higher concentrations,multiple interactions can take place, as with sensitive ribo- somes. The main difference between sensitive and resistantribosomes is that, with the former, the 1st erythromycinmolecule 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 believedto 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 antibioticwhich affects bacterial protein synthesis. Measurementsof erythromycin binding under equilibrium conditions have revealed association constants of 2.6 X lo7 and approxi-mately 8 X lo5 M-' for sensitive and resistant ribosomes

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