Pfam PF01165: Ribosomal protein S21 (family)
This entry represents small ribosomal subunit protein bS21 found in bacteria and in some eukaryotic organelles such as chloroplast and mitochondria.
Evidence suggests that, in prokaryotes, the peptidyl transferase reaction is performed by the large subunit 23S rRNA, whereas proteins probably have a greater role in eukaryote ribosomes. Most of the proteins lie close to, or on the surface of, the 30S subunit, arranged peripherally around the rRNA . The small subunit ribosomal proteins can be categorised as primary binding proteins, which bind directly and independently to 16S rRNA; secondary binding proteins, which display no specific affinity for 16S rRNA, but its assembly is contingent upon the presence of one or more primary binding proteins; and tertiary binding proteins, which require the presence of one or more secondary binding proteins and sometimes other tertiary binding proteins. The small ribosomal subunit protein bS21 contains 55-70 amino acid residues, and was thought to be present only in eubacteria, however, plant chloroplasts and mammalian mitochondria contain ribosomal subunit protein bS21 (referred to as bS21c and bS21m, respectively). Experimental evidence has revealed that bS21 is well exposed on the surface of the Escherichia coli ribosome , and is one of the 'split proteins': these are a discrete group that are selectively removed from 30S subunits under low salt conditions and are required for the formation of activated 30S reconstitution intermediate (RI*) particles.
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 . 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 .