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Extraction with 2 M lithium chloride removes a group of proteins (LiCl SP) from 50S ribosomal subunits. Both the LiCl SP and the resulting cores, which contain the remaining proteins as well as 5S and 23S RNA, lack peptidyl transferase activity, as measured by the "fragment reaction." Activity can be restored to the LiCl cores by reconstitution with LiCl SP under conditions of high temperature and high ionic strength. The LiCl SP proteins were fractionated by carboxymethyl-cellulose and Sephadex G-100, and the individual fractions were tested by this reconstitution system. Of the 18 ribosomal proteins found in the LiCl SP, only L16 is essential for reconstitution of peptidyl transferase activity.The discovery that peptidyl transferase is an integral part of ribosomal structure was of particular importance in focusing attention on the ribosome as an active participant in the translation process (1, 2). Since that time, the view has gradually emerged that many aspects of protein synthesis, such as translocation (3, 4), although mediated by nonribosomal factors, are intrinsic properties of the ribosome itself.In order to understand the peptidyl transferase reaction in terms of simple chemical mechanisms, it is essential to identify the ribosomal components involved, as well as the extraribosomal ligands with which they interact. Monro and coworkers have shown that under the conditions of the "fragment reaction," peptide bond formation is not dependent on the presence of the 30S ribosomal subunit, mRNA, intact tRNA, supernatant protein factors, or GTP (5). Although little is known about the identity of the riboso'Mal components involved in this function, it has been found that removal of a subset of proteins from the 50S subunit gives rise to inactive, protein-depleted "core" particles, and that reconstitution of the split proteins (SP) with the core particles restores activity (6). The latter observation suggested that one or more proteins that can be easily removed from the 50S subunit are essential for the peptidyl transferase function and presented a system that could be used to identify these proteins. An important step toward a molecular description of peptidyl transferase is the identification of these essential split protein components.

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