Ribosomal protein L12p. This subfamily includes archaeal L12p, the protein that is functionally equivalent to L7/L12 in bacteria and the P1 and P2 proteins in eukaryotes. L12p is homologous to P1 and P2 but is not homologous to bacterial L7/L12. It is located in the L12 stalk, with proteins L10, L11, and 23S rRNA. L12p is the only protein in the ribosome to occur as multimers, always appearing as sets of dimers. Recent data indicate that most archaeal species contain six copies of L12p (three homodimers), while eukaryotes have four copies (two heterodimers), and bacteria may have four or six copies (two or three homodimers), depending on the species. The organization of proteins within the stalk has been characterized primarily in bacteria, where L7/L12 forms either two or three homodimers and each homodimer binds to the extended C-terminal helix of L10. L7/L12 is attached to the ribosome through L10 and is the only ribosomal protein that does not directly interact with rRNA. Archaeal L12p is believed to function in a similar fashion. However, hybrid ribosomes containing the large subunit from E. coli with an archaeal stalk are able to bind archaeal and eukaryotic elongation factors but not bacterial elongation factors. In several mesophilic and thermophilic archaeal species, the binding of 23S rRNA to protein L11 and to the L10/L12p pentameric complex was found to be temperature-dependent and cooperative.
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