InterPro domain: IPR000206

Ribosomes are the particles that catalyse mRNA-directed protein synthesis in all organisms. The codons of the mRNA are exposed on the ribosome to allow tRNA binding. This leads to the incorporation of amino acids into the growing polypeptide chain in accordance with the genetic information. Incoming amino acid monomers enter the ribosomal A site in the form of aminoacyl-tRNAs complexed with elongation factor Tu (EF-Tu) and GTP. The growing polypeptide chain, situated in the P site as peptidyl-tRNA, is then transferred to aminoacyl-tRNA and the new peptidyl-tRNA, extended by one residue, is translocated to the P site with the aid the elongation factor G (EF-G) and GTP as the deacylated tRNA is released from the ribosome through one or more exit sites [ 1 , 2 ]. About 2/3 of the mass of the ribosome consists of RNA and 1/3 of protein. The proteins are named in accordance with the subunit of the ribosome which they belong to - the small (S1 to S31) and the large (L1 to L44). Usually they decorate the rRNA cores of the subunits.

Many ribosomal proteins, particularly those of the large subunit, are composed of a globular, surfaced-exposed domain with long finger-like projections that extend into the rRNA core to stabilise its structure. Most of the proteins interact with multiple RNA elements, often from different domains. In the large subunit, about 1/3 of the 23S rRNA nucleotides are at least in van der Waal's contact with protein, and L22 interacts with all six domains of the 23S rRNA. Proteins S4 and S7, which initiate assembly of the 16S rRNA, are located at junctions of five and four RNA helices, respectively. In this way proteins serve to organise and stabilise the rRNA tertiary structure. While the crucial activities of decoding and peptide transfer are RNA based, proteins play an active role in functions that may have evolved to streamline the process of protein synthesis. In addition to their function in the ribosome, many ribosomal proteins have some function 'outside' the ribosome [ 3 , 3 ].

This family of large subunit ribosomal proteins is called the L7/L12 family. L7/L12 is present in each 50S subunit in four copies organised as two dimers. The L8 protein complex consisting of two dimers of L7/L12 and L10 in Escherichia coli ribosomes is assembled on the conserved region of 23 S rRNA termed the GTPase-associated domain [ 4 ]. L7 and L12 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 [ 5 , 6 , 7 , 8 , 8 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 ].

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 homologue is found in some eukaryotes, in mitochondria and chloroplasts [ 20 ]. 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 [ 21 ].

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