Modification of Escherichiu coli ribosomes with N-bromosuccinimide (SucNBr) (molar ratio of reagent to ribosome equal to 10) is accompanied by 100 -200 %increase in poly(U)-directed polyphenylalanine synthesis, while no change in peptidyl transferase activity ('fragment reaction') was observed. The magnesium dependence of the modified ribosomes in polyphenylalanine synthesis (optimum at 11 mM) is different from that of untreated ones (optimum at 9 mM). Hybridization of SucNBr-treated and untreated subunits showed that stimulation is produced by modification of the 50-S subunit.Stimulation is the result of an increase in the rate of polyphenylalanine synthesis without activation of inactive ribosomes present in the preparation. Treatment with SucNBr is also accompanied by the appearance of a peak intermediate between 70-S ribosomes and 5 0 3 subunits in sucrose gradient centrifugation. At 7 mM magnesium the intermediate peak appears closer to 50-S subunits than at 10 mM, disappearing at 20 mM magnesium or after treatment with glutaraldehyde with a concomitant increase in 70-S ribosomes. These results suggest that the observed intermediate peak is produced by a rapid association/dissociation equilibrium between modified 50-S and 30-S subunits with decreased affinity for each other. Modification with 5,5'-dithio-bis(2-nitrobenzoate) (Nbsz) prior to the SucNBr treatment prevents the stimulation of polyphenylalanine synthesis. When the SucNBr-stimulated ribosomes are incubated with 2-mercaptoethanol or dithiothreitol the observed stimulation disappears. These results indicate that stimulation is produced by modification of ribosomal sulfhydryl groups by SucNBr. The stimulatory modification with SucNBr decreases by 5 -6 the number of -SH groups per ribosome reactive with Nbsz while no significant change in the total number of tryptophan residues was observed (tryptophan residues are the next most likely reactive groups in the ribosome).Much work has been done on the chemical modification of ribosomal sulfhydryl groups and its effects on the functional and structural properties of the ribosome. However, the results obtained are sometimes confusing and even contradictory. Treatment of Escherichia coli ribosomes with sulfhydryl-group reagents is usually accompanied by inactivation of polypeptide polymerization. In some cases inactivation reaches 30-70% and does not increase any more with further increases in reagent concentration or incubation time [1,2]. According to Moore [2], a fraction of ribosomes is susceptible to inactivation while the rest are resistant. The inactivation is located at the 30-S subunit [l]. Modification by most sulfhydrylAhhreviutions. HOHgBzOH, p-hydroxymercuribenzoate ; Nbsz, Enzyme. Pyruvate kindse (EC 2.7.1.40). (2-nitrobenzoate); SucNBr, N-bromosuccinimide. group reagents is followed by dissociation of ribosomes into 50-S and 30-S subunits, and sometimes by the appearance of a component with a sedimentation rate intermediate between 70 S and 50 S [1,3,4]. Dissociation is not observed at high ...