InterPro domain: IPR005813

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 ].

The ribosomal protein family L20, from the large (50S) subunit, contains members from eubacteria, as well as their mitochondrial and plastid homologs. L20 is an assembly protein, required for the first in vitro reconstitution step of the 50S ribosomal subunit, but does not seem to be essential for ribosome activity. L20 has been shown to partially unfold in the absence of RNA, in regions corresponding to the RNA-binding sites. L20 represses the translation of its own mRNA via specific binding to two distinct mRNA sites, in a manner similar to the L20 interaction with 23S ribosomal RNA [ 4 , 5 , 6 , 7 , 8 , 9 , 10 ].

1. Atomic structures at last: the ribosome in 2000. Curr. Opin. Struct. Biol. 11, 144-54
2. The ribosome in focus. Cell 104, 813-6
3. The end of the beginning: structural studies of ribosomal proteins. Curr. Opin. Struct. Biol. 10, 633-6
4. The role of disordered ribosomal protein extensions in the early steps of eubacterial 50 S ribosomal subunit assembly. Int J Mol Sci 10, 817-34
5. A competition mechanism regulates the translation of the Escherichia coli operon encoding ribosomal proteins L35 and L20. J. Mol. Biol. 375, 612-25
6. Escherichia coli ribosomal protein L20 binds as a single monomer to its own mRNA bearing two potential binding sites. Nucleic Acids Res. 35, 3016-31
7. Ribosomal protein L20 controls expression of the Bacillus subtilis infC operon via a transcription attenuation mechanism. Nucleic Acids Res. 35, 1578-88
8. Coexistence of two protein folding states in the crystal structure of ribosomal protein L20. EMBO Rep. 7, 1013-8
9. Double molecular mimicry in Escherichia coli: binding of ribosomal protein L20 to its two sites in mRNA is similar to its binding to 23S rRNA. Mol. Microbiol. 56, 1441-56
10. Messenger RNA secondary structure and translational coupling in the Escherichia coli operon encoding translation initiation factor IF3 and the ribosomal proteins, L35 and L20. J. Mol. Biol. 228, 366-86