<p>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 [<cite idref="PUB00007068"/>, <cite idref="PUB00007069"/>]. 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. </p><p>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 [<cite idref="PUB00007069"/>, <cite idref="PUB00007070"/>].</p><p> This entry models a family of proteins with full-length homology to the general stress protein Ctc of <taxon tax_id="1423">Bacillus subtilis</taxon>, a mesophile, and ribosomal protein TL5 of <taxon tax_id="274">Thermus thermophilus</taxon>, a thermophile. Ribosomal protein L25 of <taxon tax_id="562">Escherichia coli</taxon> and <taxon tax_id="727">Haemophilus influenzae</taxon> appear to be orthologous but consist only of the N-terminal half of Ctc and TL5. Both short (L25-like) and full-length (CTC-like) members of this family bind the E-loop of bacterial 5S rRNA. This protein appears to be restricted to bacteria and organelles, and consists of at most one copy per prokaryotic genome.Ctc of Bacillus subtilis has now been localized to ribosomes and can be viewed as the long form, or Ctc form, of L25. The C-terminal domain of sll1824, an apparent L25 of Synechocystis PCC6803, matches the N-terminal domain of this family. Examples of L25 and Ctc are not separated by a UPGMA tree built on the region of shared homology. </p>