Ribosomal protein L7/L12. Ribosomal protein L7/L12 refers to the large ribosomal subunit proteins L7 and L12, which are identical except that L7 is acetylated at the N terminus. It is a component of the L7/L12 stalk, which is located at the surface of the ribosome. The stalk base consists of a portion of the 23S rRNA and ribosomal proteins L11 and L10. An extended C-terminal helix of L10 provides the binding site for L7/L12. L7/L12 consists of two domains joined by a flexible hinge, with the helical N-terminal domain (NTD) forming pairs of homodimers that bind to the extended helix of L10. It is the only multimeric ribosomal component, with either four or six copies per ribosome that occur as two or three dimers bound to the L10 helix. L7/L12 is the only ribosomal protein that does not interact directly with rRNA, but instead has indirect interactions through L10. The globular C-terminal domains of L7/L12 are highly mobile. They are exposed to the cytoplasm and contain binding sites for other molecules. Initiation factors, elongation factors, and release factors are known to interact with the L7/L12 stalk during their GTP-dependent cycles. The binding site for the factors EF-Tu and EF-G comprises L7/L12, L10, L11, the L11-binding region of 23S rRNA, and the sarcin-ricin loop of 23S rRNA. Removal of L7/L12 has minimal effect on factor binding and it has been proposed that L7/L12 induces the catalytically active conformation of EF-Tu and EF-G, thereby stimulating the GTPase activity of both factors. In eukaryotes, the proteins that perform the equivalent function to L7/L12 are called P1 and P2, which do not share sequence similarity with L7/L12. However, a bacterial L7/L12 homolog is found in some eukaryotes, in mitochondria and chloroplasts. In archaea, the protein equivalent to L7/L12 is called aL12 or L12p, but it is closer in sequence to P1 and P2 than to L7/L12.