The binding of 6-demethylchlortetracycline to 70S, 50S and 30S ribosomal particles: a quantitative study by fluorescence anisotropy

The binding of demeclocycline (6-demethylchlortetracycline) to ribosomes and ribosomal subunits from Escherichia coli was investigated by using the fluorescence anisotropy of the antibiotic to determine the extent of binding. Binding data obtained from 70S and 30S particles differed fundamentally from those obtained from 50S subunits: the first two showed a strong, specific interaction while the third did not. In addition, all three particles possessed weak, unspecific binding sites. Computer-aided least-squares analysis of the data yielded the following numbers of sites and equilibrium constants: for 30S, n1 = 1, K1 = 2.2 X 10(6) M-1, n2 K2 = 0.029 X 10(6) M-1; for 50S, n1 = 0, n2 K2 = 0.035 X 10(6) M-1; for 70S, n1 = 1, K1 = 3.2 X 10(6) M-1, n2 K2 = 0.082 X 10(6) M-1. These data resolve current disagreement in the literature and are a prerequisite for quantitative studies of the mechanism of inhibition by tetracycline of protein biosynthesis.