When Escherichia coli 50-S ribosomal subunits are treated with increasing concentrations of urea partial deproteination occurs. Furthermore, we observed that the number of sulfhydryl groups which react with Ellman's reagent is a sigmoidal function of the urea concentration. These results are similar to those previously reported for the 30-S subunit (Acharya, A.S. and Moore, P.B. (1973) J. Mol. Biol. 76, 207-221). For both subunits we identify the proteins which dissociate (split proteins) or are recoverable in a ribonucleoprotein particle (core proteins) under the action of 6 M urea in a buffer of moderate ionic strength.
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