Dissociation of guanosine nucleotide.cntdot.elongation factor G.cntdot.ribosome complexes

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Abstract
The spontaneous dissociation of complexes containing [Escherichia coli] elongation factor G (EF-G), the ribosome and either GDP plus fusidic acid, guanyl-5''-yl imidodiphosphate or guanyl-5''-yl methylene diphosphonate was measured. It follows biphasic kinetics that can be resolved into 2 1st-order decay rates. Two classes of complexes may exist with apparent dissociation rate constants (k) differing 5 to 20-fold. The values of k and the distribution of complexes between the fast and the slowly decaying class depend on the conditions in which the dissociation occurs but not on the conditions in which the complexes are formed. Rapid transitions of complexes from one to the other class occur only when the chemical environment in which the dissociation takes place is modified. Increasing the concentration of NH4Cl or adding the antibiotic thiostrepton accelerates the decay and converts slowly dissociating into fast dissociating complexes. Addition of misreading-inducing aminoglycoside antibiotics of the neomycin, kanamycin, streptomycin and gentamicin (but not hygromycin) groups slows down the decay. For neomycin B at 10 .mu.M, this effect is due to the conversion of fast into slowly decaying complexes. A model to explain the results involving conformational transitions of the complexes is proposed.