Prokaryotic elongation factors


Prokaryotic elongation factors

In prokaryotes, three elongation factors are required for translation: EF-Tu, EF-Ts, and EF-G.

EF-Tu

The prokaryotic factor EF-Tu (elongation factor thermo unstable) mediates the entry of the aminoacyl tRNA into a free site of the ribosome. EF-Tu functions by binding an aminoacylated, or charged, tRNA molecule in the cytoplasm. This complex transiently enters the ribosome, with the tRNA anticodon domain associating with the mRNA codon in the ribosomal A site. If the codon-anticodon pairing is correct, EF-Tu hydrolyzes GTP into GDP and inorganic phosphate, and changes in conformation to dissociate from the tRNA molecule. The aminoacyl tRNA then fully enters the A site, where its amino acid is brought near the P-site polypeptide and the ribosome catalyzes the covalent transfer of the polypeptide onto the amino acid.

EF-Tu contributes to translational accuracy in three ways. It delays GTP hydrolysis if the tRNA in the ribosome’s A site does not match the mRNA codon, thus preferentially increasing the likelihood for the incorrect tRNA to leave the ribosome. It also adds a second delay (regardless of tRNA matching) after freeing itself from tRNA, before the aminoacyl tRNA fully enters the A site. This delay period is a second opportunity for incorrectly-paired tRNA (and their bound amino acids) to move out of the A site before the incorrect amino acid is irreversibly added to the polypeptide chain. A third mechanism is the less well understood function of EF-Tu to crudely check amino acid-tRNA associations, and reject complexes where the amino acid is not bound to the correct tRNA coding for it.

EF-Ts

EF-Ts serves as the guanine nucleotide exchange factor for EF-Tu, catalyzing the release of GDP from EF-Tu. This enables EF-Tu to bind to a new GTP molecule, release EF-Ts, and go on to catalyze another aminoacyl tRNA addition.

EF-G

The factor EF-G catalyzes the translocation of the tRNA and mRNA down the ribosome at the end of each round of polypeptide elongation. Homologous to EF-Tu + tRNA, EF-G also binds to the ribosome in its GTP-bound state. When it associates with the A site, EF-G causes the tRNA previously occupying that site to occupy an intermediate A/P position (bound to the A site of the small ribosomal subunit and to the P site of the large subunit), and the tRNA in the P site is shifted to a P/E hybrid state. EF-G hydrolysis of GTP causes a conformation change that forces the A/P tRNA to fully occupy the P site, the P/E tRNA to fully occupy the E site (and exit the ribosome complex), and the mRNA to shift three nucleotides down relative to the ribosome due to its association with these tRNA molecules. The GDP-bound EF-G molecule then dissociates from the complex, leaving another free A-site where the elongation cycle can start again.

EF-G is also involved, with Ribosome Recycling Factor, in ribosome recycling.

External links

*
*


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Eukaryotic elongation factors — are very similar to those in prokaryotes. Elongation in eukaryotes is carried out with two elongation factors: *eEF 1, whose α and βγ subunits act as counterparts to EF Tu and EF Ts, respectively *eEF 2, the counterpart to prokaryotic EF GGenes*… …   Wikipedia

  • Prokaryotic translation — is the process by which messenger RNA is translated into proteins in prokaryotes.InitiationInitiation of translation in prokaryotes involves the assembly of the components of the translation system which are: the two ribosomal subunits (small and …   Wikipedia

  • Elongation factor — Elongation factors are a set of proteins that facilitate the events of translational elongation, the steps in protein synthesis from the formation of the first peptide bond to the formation of the last one.Elongation is the most rapid step in… …   Wikipedia

  • Prokaryotic transcription — is the process in which messenger RNA transcripts of genetic material in prokaryotes are produced, to be translated for the production of proteins. Prokaryotic transcription occurs in the cytoplasm alongside translation. Unlike in eukaryotes,… …   Wikipedia

  • Eukaryotic translation — is the process by which messenger RNA is translated into proteins in eukaryotes. It consists of initiation, elongation and termination [http://nobelprize.org/educational games/medicine/dna/a/translation/ nobelprize.org] Translation] InitiationCap …   Wikipedia

  • Proliferating cell nuclear antigen — The assembled human DNA clamp, a trimer of the protein PCNA …   Wikipedia

  • Transfer RNA — The interaction of tRNA and mRNA in protein synthesis tRNA Identifiers Symbol tRNA …   Wikipedia

  • Eukaryotic initiation factor — Eukaryotic initiation factors are proteins used in eukaryotic translation. There exist many more eukaryotic initiation factors (eIF) than prokaryotic initiation factors due to greater biological complexity. Processes eIF is involved in include:… …   Wikipedia

  • Central dogma of molecular biology — The central dogma of molecular biology was first articulated by Francis Crick in 1958[1] and re stated in a Nature paper published in 1970:[2] Information flow in …   Wikipedia

  • Origin of replication — The origin of replication (also called the replication origin) is a particular sequence in a genome at which replication is initiated.[1] This can either be DNA replication in living organisms such as prokaryotes and eukaryotes, or RNA… …   Wikipedia