<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>The small subunit ribosomal proteins canbe categorised as: primary binding proteins, which bind directly andindependently to 16S rRNA; secondary binding proteins, which display nospecific affinity for 16S rRNA, but its assembly is contingent upon thepresence of one or more primary binding proteins; and tertiary bindingproteins, which require the presence of one or more secondary bindingproteins and sometimes other tertiary binding proteins.The small ribosomal subunit protein S19 contains 88-144 amino acid residues.In <taxon tax_id="562">Escherichia coli</taxon>, S19 is known to form a complex with S13 that binds strongly to 16S ribosomal RNA. Experimental evidence [<cite idref="PUB00004907"/>] has revealed that S19 is moderately exposed on the ribosomal surface, and is designated a secondary rRNA binding protein. S19 belongs to a family of ribosomal proteins [<cite idref="PUB00004907"/>, <cite idref="PUB00001612"/>] that includes: eubacterial S19; algal and plant chloroplast S19; cyanelle S19; archaebacterial S19; plant mitochondrial S19; and eukaryotic S15 ('rig' protein).</p>