<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> Several prokaryotic and eukaryotic proteins that are involved in the translation process contain an SH3-like domain, consisting of a partly opened beta barrel, where the last strand is interrupted by a 3-10 helical turn. This entry represents the SH3-like beta barrel domain found in the ribosomal proteins L24 and L26, the structure of which has been determined for L24 from Archaea <taxon tax_id="2238">Haloarcula marismortui</taxon> [<cite idref="PUB00030943"/>]. The 50S subunit proteins function primarily to stabilise inter-domain interactions that are necessary to maintain the subunit's structural integrity, displaying a wide variety of protein-RNA interactions. Interactions between RNA and the SH3 domains appear to be mediated by the loops connecting the beta-strands and not the beta-barrel itself. L24 uses these loops between beta-strands to contact H19 and H24. </p>